Painters do it all of the time- they take a canvas horribly gone wrong and paint right over it. I wonder how many masterpieces have been created over a work that was shmeared by an elbow or painted by a very dear, but untalented aunt. It is in our very nature to make and destroy- hence, civilizations rise and fall.
Lately I've been revisiting my mistakes. I hate throwing things away, so the other day when I was tidying up my workspace I found some early stamped postcards that I completely fudged. Now, some I've used just to write on (as space) and then have thrown in a card for friends. I can't stand wasting paper- and watercolor postcard paper is very nice. That's when I realized that there was a better way to re-use this canvas- collage!