Bryan explained well, and Johnny will explain more, but here are a few more examples:
This is a Brown/Yellow scrap. Only a few colors were printed before the card was thrown away.
scan0001.jpg
This one was nearly finished, but is missing the dark blue ink for the background. Also apparently thrown in the trash.
scan0038.jpg
This one is a "cylander transfer" which caused a back to print in reverse on the front.
scan0036.jpg
And here is a finished card that was discarded without having anything printed on the back.
scan0003.jpg
scan0005.jpg