I find it weird how most of my better drawings were started as accidents... I accidintely (I can't believe I can't spell accidentally) did something wrong that made a perfect head shape for a chibi, or a perfect looking tree stump (trees, my biggest weakness).
Next time you make a mistake, embrace it, don't erase, think of it as an opportunity to add some flair, or probably a different style to your artwork.
You never know, that mistake might lead you to your greatest masterpiece...