Trying to kill an enemy in an FPS for ammo and having that enemy blown up because some other NPC had to lob fucking grenades at it. Doesn't matter whether the NPC is an ally or another enemy, it's still annoying.
You'd think I'd be more irritated at frequently being in the radius of the grenade, but I'm usually not, unless there's no easy way to recharge my shields. It's perverse. Probably this is due to my gamer OCD where I have to collect as much ammo as physically possible, regardless of whether I actually need it or not.