I hate it in games, were the AI is so bad it runs right in front of your sniper rifle, looking down the barrel just as you hit the trigger 
I was applauded by the opposite team on Vs. mode when I "finished" the No Mercy finale with myself and the 3 AI characters (other team members all rage-quit).Legion said:Well the AI in L4D and L4D2 have some kind of aversion to shooting Special Infected sometimes.
I got hit by a Smoker and as opposed to shooting it, the AI decided to run all the way through obstacles and areas to melee me free, the insane amount of time this took meant I was downed despite having high health to start with.
This kind of thing is quite frequent.