Watched my friend try to salvage a muddy watercolor sky once. She ended up just painting the whole thing over with a dark, stormy gray, which actually looked pretty cool. Sometimes the mistake becomes the whole point of the piece, you know (but yeah, starting over is still a pain).
Actually, turning mistakes into the focus can make the art feel lazy. The stormy gray might look cool, but it's just avoiding the hard work of fixing the sky. Starting over teaches more about skill than covering up ever will.