Getting bodyblocked by teammates trying to get into a doorway. Most of the time they aren't doing it intentionally, but it's frustrating nonetheless. A close second would be a teammate running into a bouncing betty, killing me with him. Running out of black hats is awful.
Actually, pretty much any time my teammates get me killed is frustrating. Many times I shouldn't have been there in the first place, but it's easy to place blame on them when it they made it that much harder to avoid.