Gears of War 2 drives me nuts sometimes. I've been playing 'multiplayer' (that is, multiplayer fights but with computer controlled enemies and allies) a lot recently- sometimes the stupidity of your allies is astounding. Like earlier today, I was cover, firing at the other team in an open area, when one of my teammates just walks right out in the open and gets mowed down, and since he's just down, another guy goes to save him and also gets downed. In this particular area they were getting wiped out so fast I couldn't even flank around to the other side, or they'd all be dead (or only 1 would be left).
Also get annoyed with the controls sometimes. Like occasionally when I press the button to move into cover, I'll instead roll out into the open, or other times I'll want to roll and I'll end up getting stuck on a cover item.
Trying to use the lancer's chainsaw can be very annoying- it seems like, if I get hit while trying to use it, I can't raise it again (even if I release the button and press it again), which usually ends in me being obliterated by the enemy's shotgun.