Mahn Bang is a house rule that I've seen a number of Korean amateurs play where losing a game by 81 points or more counts as two losses.
I've not heard of this side rule used anywhere else and I'm fairly sure that you won't see it in any tourneys...but I found it to be an interesting rule.
Why 81 points? I'm not too sure but I'd assume that it's because 81 points is the point total for a 9x9 board, thus making a loss by 81 points or more essentially being the same as losing by a full 1/4 of a full 19x19 board. (exswoo, 12k)
Niklaus: I guess resigning is not allowed then