That things rarely go according to plan is part of running an RPG. The players will surprise you, and often this is entirely fine - in fact, it might result in a more memorable experience and a better game.
But sometimes, things go off the rail in a bad way - and you, as a GM, did not see it coming until it was too late, resulting in disaster and a non-fun experience for all concerned.
And contemplating some of my past mistakes, I am curious about your biggest GMing regrets. What went wrong, and what would you have done differently if you had the opportunity for a do-over?
Very first time playing an RPG. None of my friends had either, but I was the most motivated so got the GM chair.
We were playing Call of Cthulhu, the classic scenario The Haunting, which came in the sourcebook.At one point a character got hit by a poultergeist-ed bed, failed a dex check, and was knocked out a second story window. The dice said he lost well over his hit points on the fall. I panicked and retconned it to make him survive. I regret doing that, because it took out any threat and tension the rest of the scenario had, in what was supposed to be a horror game.
It was partially made up by that character much later activity deciding to look over his shoulder while fleeing a cultist ritual, failing a sanity check when seeing a mask of Nyarlethotep himself, and permanently losing his mind.
Admittedly I have not GMed a lot, been a player more often, but I still regret NOT killing a PC.
I've only ran one session of one game of Cyberpunk, and I also pulled punches more than I should have.