mistakes in art

In responce to Mary Marcil's question "can there be a mistake in art, which one would have to admit having made? or do the values which change overlook any mistake?" I would have to argue that there is always mistakes in art sometimes the mistake is what makes the art piece wonderful. I have heard of many stories were an artist doesn't realize that he did something or did it by accident and by doing this it ends up improving the piece of art. My question is if the artist doesn't voluntarily make the mistake then is it still concidered art even though it was not the artists intention?