Pretty sure that the last rules revision allowed you to identify a ball in a bunker, therefore if your partner played the wrong ball it was his responsibility and he'd either get a 2 stroke penalty or lose the hole depending on the format.

Previously there was no pen for playing the wrong ball from the sand as you weren't allowed (for instance) to brush away covering sand to check whether it was yours.

Rule 12 I think off the top of my head.