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?
@juergen_hubert Short Feng Shui 2 game where the player characters were all from the Mad Max future and end up in the modern day era. Had an encounter with some fake police.
I didn’t realize there’s been a shooting of a local officer recently, and one of my players was a local officer.
It went poorly in an otherwise memorable and fun game.