An impossible, improbable, occurrance that is only achieved by the lower part of our society. But if you occomplish a fail that is impossible, than in theory, it could be labled an "epic fail."
Parking your car in your roof is, in theory, an "epic fail."
a funny, very ironic happening that leaves the person in question sad or hurt, but not dead.
On CoD4, trying to throw a grenade out of a window-only to hit the window-sill and bouncing back inside would be considered a fail.