People that scream at other players to call themselves out after shooting in their direction.
BBs can swing wide or drop short at the last moment, which might be hard to spot if you are in the middle of a shooting engagement. BBs also lose energy over distance travelled, so even though you can see the BBs bounce off of someones' side, there is a chance the other player may not have even realized they had been hit. You can't call what you can't feel. The easiest solution is to move closer, and repeat firing until the player knows for certain they have been hit. Another solution would be to approach the player in the safe zone afterwards to discuss the situation if you felt it was done intentionally. When members continually do this at local fields it can drive new players away, and it seems kind of unsportsmanlike.
On the flipside of this, if you are uncertain that you've been hit, calling yourself out and complimenting the shot can keep things positive. (I do this if I am uncertain and have gotten acquainted with several great players in the safe zone after the game discussing the firefight!)
Another couple peeves are people touching/picking up/ firing your guns without asking permission first, and people that actually throw their rubber knives at other players to try and get the kill. I mean like full on throwing it while sprinting and having it knock the person's head back with the weight of the impact, and you are expecting the person on the receiving to not be angry?