I'm so close to getting a Platinum Trophy in Dark Souls and I accidentally kill an un-respawning boss named Crossbreed Priscilla without nabbing her tail for a rare weapon.
For those of you who haven't played Dark Souls, some boss' tails can be cut off and used as a weapon. One of the trophies require you to get all the rare weapons in the game Priscilla's tail included. Once you kill a boss, they do not respawn until you beat the game and start all over. The difficulty will rise with each time you beat the game on a character profile with enemies getting stronger.
On top of that, I have to PvP for a certain item required to get a miracle (a certain magic in game) for %100 completion. Boy, do a lot of players LAAAAAAAAAAAAAG. I'm on the verge of chucking my copy of Dark Souls out the window.