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?
I have many regrets but this is one that comes to mind:
I had a campaign where there were these impersonators acting like they were the players and doing bad things in their name. When the players finally confronted them, they were understandably annoyed, and so wanted to capture one as a prisoner to humiliate them.
One of the players and I was against it, but the other players wanted this and I 'thought' I could turn it into a neat story set up (as they could get some knowledge on the BBEG's plans).
Well it didn't work out as they didn't ask the NPC anything, they were mostly being weird (not sexually, just weird) with him. So in a later encounter he was unceremoniously targeted and killed by a random archer.
Issue solved right?
Well later on, they managed to see past a shop keeper's trick and realize she was upselling them on junk. Then the same players asked, well if we could capture the other guy, why can't we capture her? So they tried asking what it would take to restrain her.
I had to put my foot down and say no, they couldn't capture NPCs anymore. It took a few minutes of arguing before they relented.
From that point on, I no longer allow players to capture NPCs.