The wrong bishop
A bishop that cannot control the promotion square means a draw, even with an extra pawn.
One of the most surprising draws in chess: bishop-and-rook-pawn versus lone king is a draw if the bishop does not control the promotion square. It does not matter how much material ahead you are - if the pawn promotes on the opposite color to your bishop, the defending king simply sits in the corner and cannot be dislodged.
The defending king heads for the corner square on the same color as the promotion square. The bishop cannot attack that corner. The attacking king cannot force the defending king out. Any attempt to push the king away results in stalemate.
This comes up more often than you think. Any time you have a rook pawn (a- or h-pawn), check what color your bishop is and what color the promotion square is. If they differ, the win may be impossible even with a large material advantage.
Examples
Rook pawn with wrong-colored bishop
White can advance the pawn: a3, a4, a5, a6, a7, a8... but it promotes on a8, a dark square. The bishop on c1 only controls light squares and can never reach a8, b8, or any dark square. The defending king simply heads for h8 and sits there - the bishop cannot drive it away. White cannot force checkmate. This position is a theoretical draw.
Key terms
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