Well, definitely not in first-person shooters.
But back when I played WoW, I came across this level 5 questing in the Undead starting area (I was on my Alliance Rogue character), minding his own business, inexplicably flagged for PvP. So I killed him. Then later I saw him again, still just questing and minding his own business. So I killed him again.
Then later I saw him talking to this level 60 rogue who promptly disappeared. So I killed the level 5 and was promptly attacked by the level 60 Horde. As we were fighting, the level 5 respawned and started helping the Horde guy out. And then I killed the level 60. And then the level 5 ran away, so I chased him down and killed him.
I felt pretty bad after that.