Good examples of absolutely useless AI in games?
Tekken 6's Scenario Mode features a level with high platforms (Bruce is the end boss). I've seen Alisa (who's computer-controlled) fall off that platform six times in a row, which doesn't really suit the idea that she's this all-powerful robot. That stage is very close to impossible to beat if you're fighting alone (the time limit is too short), so if she gets knocked off the platform you might as well give up and try again.
What other games feature a colossal AI fail?
