Test prints are a common practice among screen printers and poster artists. A printer will usually keep a few sheets of stock around to make sure the screen is clearing properly or to check registration. They're essentially the protracted layering of several different colors from various prints on a single sheet of paper. The result is a unique print, often rife with aesthetically pleasing happy accidents. Test prints are one of my favorite parts of screen printing. Below are some that I recently put up for sale in the gigposters.com classifieds. Just for fun, I will be including an original drawing to ship out with each of them.
One more thing--check out one of my favorite posters of all time by Nick and Nadine at Sonnenzimmer. I can't say enough good things about this print. They truly are blurring the line between fine art and illustration in this thing we call the rock poster.
Nick and Nadine are the founding members of The Chicago Printers Guild. Also, Nadine speaks German!