Inventing new rules in a game, specifically to prevent you from winning when you're about to win.
He pulled a brodigans again!
Revealing new rules to a game (which you already knew) that can drastically alter the balance of a game in your favor.
Specifically to prevent you from winning when you're about to win.
Dave: You can't place a tile on the board
John: Why not, we've been doing that the whole game?
Dave: Bitch please, It says it right here in the rule book.
John: Brodigans!
To "accidentally" forget a key rule until a vital moment in a board or card game, usually benefitting people of the same name.
He only won the game because of a total Brodigans.