Your nick makes crystal clear that this experience traumatized you badly!
Stealing a piece while the opponent's not looking at the board

As a matter of fact, I do. True story from local tournament:
Man played a move, got up for a stroll and when he sit he discovered that a NIGHT is missing. Not only that,but the guy wrote a blunder move on BOUTH sheets(papers) He got knock out, straight away. We still smile when remember that one.

Once my chess friend cheated. When his queen was on d1 his he accidentaly made the queen on d2. So he took the queen on f4

I heard about a guy who stole a rook, not from his opponent but from the board next to him.
What had happened is he had already lost one of his rooks, so he castled long, so to speak, snatching the rook from the board adjacent as he did.
these are unbelievable stories. How in the world do you not notice your N is missing or your opponent picked up a rook while you went for a coffee?

What he should have done was to put his king on the next board, that way he could never be mated.
New chess variant?

Your nick makes crystal clear that this experience traumatized you badly!
lol this somehow made my day

It happens often in live chess. Once I lost my rook for more than 10 moves. It wasnt there and all of a sudden it appeared. I don't know if it was missing on my opponent's board as well...

I had friends that tried that a few times growing up, or they would remove all the pieces, because they knew they were losing. However I always immediately noticed the board was different or just put all the pieces back the way they were. I would laugh at them.
I was playing in a simul against an IM -- probably about 25 boards. I relealized I was going to lose a piece when he came around again. My friends and I thought we'd see if we could fool him by bumping a piece to another square where things were safe. Mostly we just wanted to see if he'd notice.
He noticed immediately. I imagine our giggling and smirking didn't help us, but I'm pretty confident he couldn't be tricked anyway.

My friend was in a drawn endgame, and got up to go to the restroom. When he came back, he saw that his opponent had played TWO moves.
My friend didn't say anything though. Because after those 2 moves, he was actually winning the game!

Sometimes i play OTB with the son of my cousin, now 19, whom i win about 90% of the time
The other day he came to my place and afer losing 3 games (and not listening to my advice) he insisted on playing one more, while i was tired and not interested, but ok, i said yes once more
After a move threatening his Knight with a pawn i went to the bathroom, and when i returned i saw that one of his pawns had proceeded 3 squares forward, threatening one of my own Knights
I asked if he had made a move and he said no...
I was somewhat upset with his lie, i don't think he realised it though... (i was a little upset with him anyway...) i didn't discuss it with him at all after this, i just told him to make his move then, and proceeded to win one more game
I'm not sure what i should have done - maybe i didn't handle it the best way?
Once when I played in the local club a pawn fell down on the floor and when I bent to get it my opponent got one of my pawns. However, I only realized that the pawn is missing 6-7 moves later. Do you know if anything like this has ever happened at high level tournaments?