[SE] Selling fish to a fisherman

[Gigermann]

…or selling ice to an Inuit, talking the headsman into letting you off the block, etc.

Social Engineering talks about throwing away the book for some situations, but as a GM, I usually dislike arbitrarily deciding things are impossible just because I think they should be. I suppose you could just assign a -10, but sometimes there's a Face Man that can absorb that kind of penalty—and you don't necessarily want to take that ability away from him, but you don't want to be too ridiculous either. Anybody have some tips or past experiences with the "impossible Influence situation" they can share?