My current pet peeve is that when you heal or buff team-mates in PvP, you turn to face them, often leaving your back exposed. While it kind of makes RP sense that you would face the person you were helping, the reality of PvP is that when your buddy is about to die, he will go for cover if he's able, which very often leaves you pivoting and showing your flank.
Another regular annoyance is when I'm getting focussed so I start healing and buffing myself, but midway through the guy I was targeting switches targets, so I end up healing them instead. While I can deal with "accidentally" healing a team-mate (they were getting shot so they probably need it too), getting spun around suddenly sucks.
I can work around the suddenly turning my back, but it's frustrating when it happens, and the game just shouldn't work that way.