This incident perhaps does not constitute a direct experience with a paranormal force, but I've never been able to find a rational explanation for what happened.
I was a student at the University of Glasgow, Scotland from 2004 until 2009. In 2005, I moved into an apartment in the west end of the city with a friend, whose parents owned the place, and another friend of his from university. We stayed there until graduation.
The following occurred in February, 2007, some 18 months after I had moved in. It was a weekday, and I had just one class to attend in the morning. My flatmates had already left, so I locked the door on my way out. My class lasted less than an hour, and I stopped only to pick up a bite to eat on the way home, so I doubt that I would have been out for more than say, an hour and a half.
A strange thing happened on my return, however. I opened the building door as normal, but then found that my key to the apartment wouldn't fit into the lock. I couldn't even get it half way. Now, I carried that key seperately, on an extremely distinctive handmade keyring, so there was no chance I had just picked up the wrong one. I had also used the same key to lock the door that morning.
I'd heard that temperature changes can sometimes cause locks to contract or stiffen, and this being the Scottish winter and me being far from a handyman, I decided this was a fair enough explanation and gave up trying to throttle the lock. I called my flatmate, who came shortly after and had no trouble opening the door with his own keys.
I didn't appreciate how odd the situation was until my flatmate tried my key out for himself, and pointed out that it was completely the wrong shape and size to fit the front door. It couldn't have been the same key I had been using every day for a year and a half, or even that morning - it was far too small for the lock. Stranger still, we had all the flat keys cut at the same time when we moved in, at the same locksmiths, and yet the key I was carrying was inscribed with a different name than the others, and looked much older.
I had never had trouble with the lock before and neither had my flatmates, who had both borrowed my key on occassion. We spent quite a while that evening speculating on how this could have happened, but could not find a satisfactory explanation. It was as though in that short hour and a half that I was out, someone had taken the keyring from my pocket, slipped the key off and replaced it with a random one. All without my noticing. This was totally absurd, but then so was much of what we came up with. It seems my old key just vanished into the same place that this new one had appeared -- from nowhere.