Pointless mandatory difficulty spikes that you cannot be prepared for in any way, and that MUST be completed in order to progress in game. I spent well over an hour struggling on Max Payne 1 with those stupid "Nightmare" sequences in which you do a platforming sequence for some reason across a path of blood, which is just barely wide enough to stand on, and if you move even slightly off it, you start the whole thing again and hear a baby screaming. Being told in A Dance With Rogues that, after sneaking around the entire game, I now have to fight a black dragon with my equipment designed to let me sneak, and I can't go grinding because you don't get experience for fighting, but instead for sneaking/picking locks/disarming traps/ect.
Please, game designers. Think about what you're doing, and cut the (undealable) difficulty spikes. And grinding too. If your player has to grind, you've usually paced the game wrong.