As said before, the really infuriating friendly fire is when it is so blatantly obvious that you're on the shooter's team such that they must have chosen to shoot you anyways.
I was standing, popping shots off with my pistol, when I turned and put my back to the cover I was at. I started to reload my mags, when a single shot hits me dead center of the chest. No other shots were fired, just this single one that was clearly for me. Standing in disbelief, I hear a half-hearted "sorry" come from the only cover behind me, a trench 30 feet away. There were no trees or anything that could have possibly made this guy think I was on the other team, plus my ribbon was CLEARLY visible to everyone back there. He was not shooting past me, because he only took one shot at my center of mass.
I will admit I lost my cool at this point and swore as I walked off the field, along the lines of "this is bullcrap, shot out by my own freaking team, AGAIN!" due to being shot out by my own team in 3 out of the last 5 games I played.