Odd Chess Rules

Over the years I've come across a number of "odd" chess rules. Rules that you won't find in any official FIDE Rule Book, but ones which your opponent believes (or even insists) are real chess rules. They are usually supported by the claim that "my father told me" or "I've always played this rule".
My own first experience with "alternative" rules was soon after I learned chess, at the age of 6 or 7. A schoolfriend who taught me claimed that pawns couldn't capture on their first move. This lead to various attempts at smothered mate, usually involving the plan Nb1-c3-e4-d6# This all went well until I tried it on my father, who both took my knight, and taught me the correct rules of chess.
Since then I've come across a number of "rules". Here a a few that I can remember (all of which are untrue btw)

Promotion (All ways a rich vein!)

• You can only promote to a piece that has already been captured
• When you promote to a queen you have to put it back on its starting square
• If you promote the a pawn you must take a rook, b pawn = knight etc (What happens with the e pawn? Two kings?)
• You have to leave your pawn on the back rank for a move before promoting it

Castling

• Can't castle if your rook is attacked
• King goes to h1, rook to e1

En-Passant

• Pawns can capture pieces en-pas
• En-pas works any time a pawn passes another

and finally my favourite (courtesy of a recent Geurt Gijssen Chess Cafe Column)

• If you only have a king left, but you manage to get it to the other end of the board, you add an extra rook to your side.