By far the most annoying bug is the one that randomly deletes your hud when you get revived. Map, crosshairs, hitmarkers, friendly markers, everything.
I spent about five minutes one day running around putting at least seven bullets into every person I came across, because their being alive at the end of it was the only way to tell if they were good or bad. Or I could have looked at the model, but I'm far too noob for that crap.
The only way to get it back is to be revived again (surprisingly hard to achieve on demand), or alt-F4 the game.