For me this would have to go to the otherwise excellent Gears of War 2 in both campain and multiplayer. First off in campain as Yahtzee already mentioned the friendly AI has a frustrating habit to leave you to die. And in multiplayer death matches when you are the only human player and the rest are bots (don't have live at the moment) there is nothing worse than dying and having to watch your AI team struggle to fight while unable to even figure out where the enemy are.
What is the dumbest AI you ever encountered and how did it effect you as a gamer?
What is the dumbest AI you ever encountered and how did it effect you as a gamer?