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?
It's a reference to D20 on a Bus, where Katie Marovitch calls herself Dragon Master to troll famous DMs like Mark Mercer, Brennan Lee Mulligan, Aabria Iyengar and Jasmine Bhullar: https://m.youtube.com/watch?v=plGNmsegcJc
...and, apparently, everyone else who has ever DMed in their lives.
That's a nice side effect. 😄 But I think her focus was mainly on messing with Brennan.