In some games, we have AI teammates. This should be a big help, right? A lot of the time, it's not. Some AI are downright terrible.
Right now, I'm playing Mass Effect and wow do your teammates stink. They like to randomly shoot at walls during a fight, wasting the ammo charges in their guns.
Left 4 Dead probably has some of the most frustrating AI. In fact, it's a game best played with at least one other human.
There are other crappy AI teammates like in Halo that make you wonder if they're actually on your side.
For a complete list of my favorite terrible AI teammates go here