I missed a very simple and quick win in this position in a tournament last weekend (White to move).
The game began as an English Hedgehog where I was very intentionally trying to do my best Ulf Andersson impersonation.
I had spent the whole game implementing the plan of controlling the d-file, putting an iron lock on d5, and ruling the light squares. Sometimes I get so caught up in the positional/strategic side of things that I forget the simple stuff.
A few moves later we ended up in this position (Black to move):
I got up and went to the restroom, thinking I have finally wrapped up a very intense game. When I returned to the board, my opponent's queen was on c5!
I was in shock, and I sat there thinking "How on earth could I have missed that?" I have had some bad tactical lapses in the last 3 months, and I just assumed that I had somehow screwed up again, not even considering that my opponent might have played an illegal move. It flustered me, and we ended up with me swindling a draw in a knight vs bishop ending.
I did not discover this until the next day when I was showing the game to a friend (ocean64 on the site here). I thought then that perhaps I had messed up the notation, and that my memory was faulty. I asked my opponent if he would verify the score with his scoresheet, and he had the same thing I did!
He was very embarrassed- we both were, for sharing a hallucination- and I am sure that he did it by accident, as he could easily have said that he did not have the score.
It was a very intense game, and we were one of the last boards in the room still playing when it ended. It is most certainly one of the most bizarre games I have played.
Moral: It's not always the guy going to the restroom that you have to watch!