TIL

The modern Chess rules had some unintended technicalities even until the 1970’s.

In particular, until the rules were amended, it was possible to read the castling rules as allowing a pawn promoted to a rook to then vertically castle with a king on its original square — i.e. e8=R ... o-O-o where the king moves e1 to e3 and the new rook moves e8 to e2?!. This was because the rules only specified that the rook must not have moved previously.

This Reddit post has a “puzzle” where this rule change affects the game result.

Wikipedia lists these under joke chess problems as if they were even then already unrealistic to consider outside of in a joke, but still interesting.

(There’s also a second “funny” or “interesting” puzzle there where the rules didn’t specify that promoted pieces needed to be of the same color.)