In folktales we find stories of who wandered from our everyday world into a magical realm, where the environment they find themselves in is strangely unfamiliar, and the inhabitants, although human looking, also display strange characteristics. These stories tend to get embellished with time, and end up as magical fairy-tales.
However there are also personal experiences from past centuries that were written down as told by the person himself. In the early 1900's many folktales were gathered by certain researchers in order to preserve this part of cultural heritage. I read a couple of these books that dealt with the region of Flanders in Belgium, and Holland. Also, because we were entering a period in which all these tales were dismissed as pure fantasy, and only that what scientists could repeat in their laboratories was the only true reality.
Reality does not care about what scientists think. Strange, unusual things happen to people all the time, in the past and in the present. One kind of phenomenon that one can find repeatedly in the old Dutch folktales, is that of an unsuspecting person walking through the fields, on his way home, noticing a strange barn in a field that he had never seen before. Upon entering he finds himself among a brightly lit space with many fair looking people dancing on heavenly music. He usually refuses and leaves the barn again. When he arrives at home many days have passed. The barn is of course not there anymore.

Several years ago I read an almost identical story from a Brazil, only a decade or two ago. Two men were walking through fields when they spotted a barn that they had never seen there before. They heard music coming from the barn, so they decided to investigate. They opened the door, and saw fair looking people dancing on the music. One of the two men wanted to enter and put his arm through the door opening, but his friend, being more cautious, pulled him back. That didn't fare well, because the man's arm got immediately numb, and he couldn't use his arm anymore. They left the place and went to the hospital where doctors could not find any cause for his now paralyzed arm. It took many months before the effect wore off, and he finally gained complete control over his arm again.

It seems that sometimes alternate realities or dimensions can open up into our physical world. It also seems that such openings are often initiated by beings of those realms.
I found an interesting story in the book The Confessions of Aleister Crowley, An Autobiography, which is an extensive account of Aleister Crowley's life, written by himself. This is not the place to go into the often misunderstood character of Crowley. On page 155, Crowley accounts an experience that happened to Oscar Eckenstein (1859-1921) who was an English rock climber, and friend, mentor and fellow mountaineer of Crowley:

This story is briefly as follows. One night after being attacked in the streets of Soho, or the district between that section of Oxford street and the Euston Road, he determined, in case of a renewed assault, to walk home by a roundabout and unfamiliar route. Somewhere in the neighbourhood of the Caledonian Road he thought that he was being followed --- it was not late at night and somewhat foggy. To make sure, he turned into a narrow passage on to which opened the gardens of a row of houses, in one, and only one, of which lights were visible. The garden door of this house was open and he dodged in to see whether the men he suspected were following. Two figures appearing at the end of the passage, he quietly closed the door behind him with the intention of entering the house, explaining his position and asking to be allowed to leave by the front door. The door was opened by a young and beautiful woman in fashionable evening dress. She appeared of good social position and, on his explaining himself, asked him to stay to supper. He accepted. No servants appeared, but on reaching the dining-room --- which was charmingly furnished and decorated with extremely good pictures, Monet, Sisley and the like, with sketches or etchings by Whistler, all small but admirable examples of those masters --- he found a cold supper for two people was laid out. Eckenstein remained for several hours, in fact until daylight, when he left with the understanding that he would return that evening. He made no note of the address, the street being familiar to him and his memory for numbers entirely reliable. I think that he was somehow prevented from returning the same evening; I am not quite sure on this point. But if so, he was there twenty-four hours later. He was surprised to find the house in darkness and astounded when no further inspection he saw a notice "To Let". He knocked and rang in vain. Assuming that he must have mistaken the number, unthinkable as the supposition was, he explored the adjacent houses, but found nothing. Annoyed and intrigued, he called on the agent the next morning and visited the house. He recognized it as that of his hostess. Even the lesser discolorations of the wallpaper where the bookcase and pictures had been testified to the identity of the room. The agent assured him that the house had not been occupied for three months. Eckenstein pointed to various tokens of recent occupancy. The agent refused to admit the conclusion. They explored the back part of the premises and found the French windows through which Eckenstein had entered, and the garden gate, precisely as he had left them. On inquiry it appeared that the house was vacant owing to the proprietor (a bachelor of some sixty years old, who had lived there a long while with a man and wife to keep house for him) having been ordered to the south of France for the winter. He had led a very retired life, seeing no company; the house had been furnished in early Victorian style. Only the one room where Eckenstein had had supper was unfurnished. The agent explained this by saying that the old man had taken the effects of his study with him to France, for the sake of their familiarity. The mystery intrigued Eckenstein immensely and he returned several times to the house. A month or so later he found the two servants had returned. The master was expected back in the spring. They denied all knowledge of any such lady as described; and there the mystery rests, save that some considerable time later Eckenstein received a letter, unsigned, in evidently disguised handwriting. It contained a few brief phrases to the effect that the writer was sorry, but it could not be helped; that there was no hope for the future, but that memory would never fade. He connected this mysterious communication with his hostess, simply because he could not imagine any other possibility.

A little bit later Crowley remarks:

Talking the whole matter over with my guide, philosopher and friend, Frater O.P.V., he finds the whole story extraordinarily gripping. He finds the situation nodal for the spirit of romance. An extraordinary number of vital threads or "nerves" of romance. He attaches great significance to the failure of Eckenstein to keep the appointment. It seems to him as if the whole business were a sort of magical ordeal, that Eckenstein should have been awake to the miraculous character of the adventure and kept his appointment though hell itself yawned between him and the house. The main test is his realization that the incident is high Magick, that if he fail to grasp its importance, to understand that unless he return that night the way will shut for ever. He suggests that by failing to appreciate the opportunity at its full value he had somehow missed the supreme chance of his life, as if the "wrong house" were the gateway to another world, an inn, so to speak, on the outskirts of the City of God.

It is really interesting that he went into a house, which looked normal inside, conversed with a woman, and even had supper with her, only to find out that all this did not exist when he returned later on. I find it intriguing that a supper for two was laid out in the table, as food or drink seems to be often central in the other stories I have read over the years of similar experiences. I wrote about such an experience in my article of Strange Encounters of the Unusual Kind, called Reality Shift. In it a Dutch psychic, a brother and sister-in-law, when traveling through France, visited a cafe aside a road to have some coffee. Several strange things were noted but not fully comprehended. After leaving the cafe, and being back on the road again, the strangeness finally dawned on them, and they decided to go back but could not find the cafe where it should have been. The entire story is much more detailed in my article, click on the above link.  

A similar story to the Dutch psychic is told by Brad Steiger in Entering The Mysterious Dimensions Of Shadow World. A person contacted him to tell the following story.

Mr. R. W. Balcom of Live Oak, California, contributed a personal experience shared with his wife. According to Balcom, the two of them were traveling to Lake Tahoe during the early morning hours. A few miles east of Placerville on Highway 50, they stopped to eat at a quaint and rustic-styled restaurant, which neither of them had ever noticed on any of their previous trips to the region. The food was excellent, and the waitress and cook were so friendly that the Balcoms truly meant their promise to stop back again. They tried to do so on their return drive from Lake Tahoe, but the restaurant was nowhere to be seen. The Balcoms traveled that route three successive weekends in 1962, searching for the friendly little restaurant with the good food that had simply vanished into nothingness. "Since then we have journeyed over Highway 50 to Lake Tahoe many times," Balcom concludes, "never again finding the little restaurant.  

Again, this story centers around food and drink place.  

We have been led to believe by our so-sacred scientists, who know everything, that our physical reality is composed of very solid physical matter obeying very strict, unalterable physical laws. I think this is not the case at all. We know from quantum theory that the underlying structure of what we call the physical world is not based on solid 'billiard ball'-like particles, but on wave-packages of energy. This would imply that when one has a profound understanding of this basic energy structure of reality, one can alter it at will and change the reality of one's environment. I think certain beings have this knowledge and are apt at altering reality. This could be a change in our physical environment, or creating a entrance into another dimension/world.

While in some instances one could come up with a reason why someone would have such a reality shift, in other other instances it just seems to defy any logic. Maybe logic or reason does not have anything to do with it.  

A British reader of the the website Phantom and Monsters send in his experience into an altered reality (notice that in this case a drink is also central):

I was 15 and living in a small town in England. It was a sunny and a warm, Sunday afternoon. With nothing much to do, my girlfriend and two neighbour boys, a couple of years older than we were, decided to go for a walk (No romance, just friends). We walked for a few miles along the river bank and we were feeling jolly but soon felt rather hot and thirsty. We came to a small wooded area and decided to explore that in the coolth of the trees. We came out of the woods into a clearing where there was an olde stone public house. Low roofed, thatched, which was unusual where we lived. It had a wooden bench outside. We sat there for a few minutes, counted our pennies and found that we could go in and get a drink of some sort. The boys decided on beer, so in we went. I remember vividly the wood planked floor, the small paned windows with sunlight streaming through, lots of dust motes in the air, which gave the interior a mist like greenish atmosphere. I also remember that there were two men sitting on a crude bench beside a thick wooden table. One old man with a pipe was sitting by a huge fireplace with a roaring fire, I thought it odd that there would be a fire on so warm a day. The boys went to the bar and asked for a beer and lemonade. The barman just stared at them as if they were speaking a foreign language. He just stood with his mouth open holding a metal tankard in his hand and not moving. The boys looked around and one of then pointed to a small cask at the bar. 'Can we have 4 ciders then please?' he said. The barman seemed dazed or ill, we didn't know which, but he took 4 battered tin pots down from a hook above his head and poured out the cider. One of the boys asked how much. The barman shook his head and waved us away. We quickly drank the cider, it was very strong, and no doubt illegal for us. But we put the cups on the bar, said thank you and left. The other men who were there never looked at us nor made eye contact. The whole place was very basic, and quaint, but we were in rural England after all. I can't remember there being a name on the pub. It was really only one or two small rooms, with a low roof, large oak beams and 2 windows and a door. We felt refreshed and we made our way back home. The following week, again it was a lovely summer's day and we were playing a ball game, when we thought we would go for another walk, and have some illicit cider. Off we went. We walked for miles almost to the next town, but we found neither the small wood nor the little pub. We searched the area for several weekends until it was time to go back to school. We never even found any ruins or stones or stumps to show that there had once been a wood or a pub. I even went to the library to see if there was anything like that on old maps, but I never found a thing. So where had we been that sunny, Sunday afternoon? We'll never know. This was 50 years ago, and I've never told anyone about this before. Sincerely, ML  

Here is another story, by Daniel Bigham (again a drink is central), from the comment section of an article on the Mysterious Universe website:

I have a strange tale from my childhood when my family was living in Nevada (one of a few such incidents) that was similar to this one. My Dad took us on vacation to Las Vegas, and around the town of Ely, my sister and I became nauseated. We stopped at a drugstore that was quaint, looked old but was cheap. The owner gave us "coke syrup" for about 50 cents, sent us on our way with good cheer and told us to stop by on our way back to let him know how we were doing. The syrup helped us, and after our visit to Vegas, we tried to find this place again... but it was not there. Confused, my father asked the townspeople what happened to the store. They chuckled and told us that the place had burned down 100 years earlier! Some of them also mentioned that our story was rare but not entirely unique. I have puzzled over this for more than 30 years, and this article comes really close to explaining (or at least offering) some kind of reference to what might have happened.

From the many stories I have read of people experiencing alternate realities or dimensions, I think there are a lot of places on the planet that allow access to other dimensions or worlds, or provide for a profound change in the environment. Sometimes people enter into these portals, gates, vortexes or whatever you want to call it, by accident. Sometimes their experiences show clear signs that other beings intentionally led people into this altered reality or dimension for purposes that are not quite clear. Maybe there is spiritual lesson in it, maybe it is an initiation, or maybe these beings just have fun with unsuspecting humans.  

Brad Steiger in his article Entering The Mysterious Dimensions Of Shadow World, writes about an interesting letter he got from someone who seem to know a lot more about at least the existence of dimensional gates. It gives a lot of food to thought:

In February of 1971, shortly after an article of mine on Time Travel had appeared in Saga magazine, I received a letter from a gentleman named Al Kiessig, who claimed the ability to walk through "doorways" between dimensions. In subsequent correspondence, Kiessig, whom I found to be an open and sincere man, shared numerous extraordinary experiences.   It was in Missouri that Kiessig found the "West Door," the door of evil, and the "East Door," the entrance into the Spirit World. According to Kiessig: "At the West Door, the wanderers of the spirit world can leave and enter our world clothed so as to be seen as one of us--and no human eye can detect the difference. ''There are two places, one in Missouri and one in Arkansas, where I walked into this next door neighbor of ours. It is very silent. It looks like our world, but there is no sound, no wind, no sun, even though it looks like the sun is shining. "In the state of Missouri I found two fields that had doors, or what I call 'vortexes': No matter where you walked you would come back to your starting place, and if you hit the center of the vortex, then you would come out from a mile to two miles beyond the place you entered in a section that would be unrecognizable to you until you stopped and regained your inner balance. Then the surroundings would gradually become familiar. "Each door is different, but it is my belief that if one could recognize these door openings, one could pick the door in Arkansas that would permit me to step into your front yard in Iowa. "In the region of the Ozarks, it was nothing for me to see into this other dimension. I could not enter, but I could see into it, as if through a large window; and I could see the people, live people, who entered our world or dimension, using the same mode of transportation so as not to give themselves away as aliens. The question is who are they? "I have entered these 'doorways' while driving and saved myself hundreds of miles of driving. Unfortunately, the reverse has also happened to me. "Some of these doors to other dimensions open like an elevator door with no elevator there to step into. Others open into a land of no life. Some take you back into the past, and some take you into the future on this world. Then there are doors that open into chambers that send the body to a distant star. "This world we know as Earth is not the only world inhabited by people like us. We must keep our minds open wide. "There is no way that I can prove any of the events stated in this letter," Kiessig closed in one of his long letters to me. "My word, which is well known hereabouts to be good as gold, must do." Is it possible that some men and women may have a peculiar psychic make-up which permits them to transgress the boundaries between this plane of reality and other dimensions? Can these men and women be possessed of abilities which enable them to travel to realms of being normally unobtainable to those in the physical body? As Al Kiessig admitted, he had only his word to substantiate his claims, and even though his friends and neighbors in Arkansas might swear by his promises and his oaths, such testimonials do not stand up well under the critical scrutiny of the scientific testing laboratory.