When it's really funny.
This is the same principle as when we were sitting in the college cafeteria howling with laughter (on dozens of occasions), and somebody would manage to gasp out, "That's really mean." And somebody else would say, "Yah, but it's funny." And on we'd go.
Really funny doesn't excuse everything. But on the flip side, really funny doesn't have room for everything within it, either. A simple-minded "you suck you suck you suck" is hardly ever really funny.
Okay, back to it.