When I'm painting, I usually have a little scrap piece of whatever paper I'm using at the time as an area to test out the colors before I lay it down on the actual piece. Sometimes these paint marks turn out more interesting than the actual piece.
Also, have you guys ever drawn random lines or scribbles and given it to somebody else to draw something using those lines? And swapping it back and forth? I don't know if that has an official name or anything but I love doing that.
Anyway, last night I was feeling silly and started doodling on my paint marks from the swine flu piece. It was a lot of fun! Unintended (unintentional?) shapes and patterns are so interesting.

My favorites are the three guys on the right: the dog (?), the cat, and the guy with a turban-esque headgear.