If it is in a tournament, it could even happen for reasons not related to the position itself. Let's say you have a winning position, but you are in severe time pressure. You really need a few moments to stop and think to figure out possible stalemate swindles your opponent might have.

You are about to promote to a queen. You realize that both of your bishops are still on the board, and promote to a bishop instead of a queen.

That lets you stop the clock and summon an arbiter to go find another bishop of the right color. While the arbiter if off dealing with that, you get your much needed thinking time.