Invisible walls. Kind of ridiculous how on every map there are numerous places where a player can appear to be peaking out from behind cover but they are impossible to hit. Don't tell me I'm the only one who gets annoyed by this at least a few times per game.

Yeah, I have had this problem. I especially hate it on the Escort Bot. Almost impossible to cover fire behind that thing.

or the see-through rails that grenades bounce off of?

I tried shooting inbetween metal rails and my bullets hit the space like it was a wall. Good thing I'm a medic or I would've died!