Generally with all of the Call of Duty games, the game is not content with having enemies do more damage and have higher intelligence when you turn up the difficulty, it feels compelled to blast your allies with nuclear radiation, rendering them completely braindead.
But if want to know one that is truly insufferable, it would be Sheva for Resident Evil 5. You better treasure your first playthrough with this woman, because the instant you select "New Game+" she will have developed so much sexual tension with Chris that she will have forgotten to help you properly. When you are DYING she decides to take 5 seconds to see if you're really on deaths' door before maybe pressing that invisible "B" button to revive you. Or if you're if you under attack by an enemy (HELP status) she will stand there and just aim her pistol (instead of a more useful weapon she has more ammo for) as if she wanted to say "I don't think you really need my help."
I've been hearing people complain that she is too quick to heal you, and I envy those people. I'd rather have her blow all my healing items on the slightest booboos if it meant that she would HELP or revive me RIGHT WHEN I NEED IT.
To all Sheva fans out there, I complement the fact that she doesn't waste ammo, and that she actually aims at the right enemies instead of lesser threats, but this is unacceptable.