Yesterday I posted a puzzle I found it in an issue of the Marshall [Club in NYC] Spectator.
White to move and mate starting 3. 1.e7…
There are several ways to mate black. However, the most fascinating line is
if 1…Kxf3 2.e8=R and now if 2…d4 3.0-0 mate BUT!!!! if 2…Kg2 3.0-0-0-0 mate.
What on earth is O-O-O-O?
It is a castling move! At the time, the FIDE rules said that a King that had not moved and a rook that had not moved could castle. In this case, the promoted former pawn, now a rook, has not moved and the unmoved king are in the same FILE, e, not the same RANK, 1. So, according to the laws of chess at the time, they could castle. Now, that cannot be done. The rules say they have to be on the starting RANK.