The old school joke puzzles

 The key move? b8=black knight!  That's the only way that black can't capture. Soon after this hypothetical situation was proposed the rules were changed to only allow for promotion of the same color.

1 e7 kxf3 2 e8=R! Kg2 3 O-O-O-O#!!! (the king moves to e3 and the rook to e2 in a vertical castle.  Since the king and the rook were in their original positions, this move was legal)]

Not surprisingly, this rule was changed too.