Opposite-Coloured Bishops

When each side has one bishop and they travel on opposite colours, endgames get very drawish — even being a pawn or two up often isn't enough, because the bishops can never challenge each other. Oddly, with rooks or queens still on the board, the same imbalance can favour the attacker.

Source for further study: Müller & Pajeken, How to Play Chess Endgames.