Involuntary collaboration refers to a project contributed to by multiple parties in which at least one of those parties did not intend to be working with the others. Examples include things like covers of songs, spoofs of movies, and in the case of the pictures above (painted in part by Chris McMahon), unwanted landscape paintings bought at yard sales, which he then painted monsters into. I think they’re brilliant, and would be a perfect DIY project to hang in your kid’s room, or send to your grandmother.
A similar process (seen below from The Monster Engine), has adults upgrading children’s drawings of monsters to add realism. Sometimes the toughest thing for an artist can be just sitting in front of the canvas wondering what to paint, or going through all the trouble of painting a whole landscape when what they really want to paint is a big hairy monster, and it can be hard to throw away original artwork, even if it is boring.
Update: It looks like more people have been picking up on this idea.
Dave Pollot and his thrift store paintings: