D&D
#DND as the characters in my campaign get higher level, they get more and more able and willing to derail DM plans. Just last session, I *thought* I had planned for them to go through a dungeon to fight a boss. Instead they refused to enter the dungeon and used Sending messages to lure the boss out, skipping the whole dungeon.
But DMing that kind of stuff is actually kind of fun. Gotta roll with the punches...