I break ghosts down into four categories: wet sheet transfers, mis-registration (i.e., color shifts), roller transfers, and scraps/test prints. I tried to include examples of my cards where possible. There are lots of examples of T206 ghosts in this thread:
http://www.net54baseball.com/showthr...ighlight=ghost
Wet sheet transfers can be an image of an ad back that has transfered onto the front of a card (sometimes years after printing) or can be the colors from the front of a card on the back. The colors sometimes form a ghosty image but they frequently just look like colors. (Leifield, posted here, is a mix)
Misregistration ghosts happen when one (or more) ink layer(s) is shifted to the side while the rest are where they should be. The image changes a lot depending on the color that is misregistered and how far off it is:
http://www.net54baseball.com/showpos...&postcount=260
http://www.net54baseball.com/showpos...&postcount=281
http://www.net54baseball.com/showpos...&postcount=291
http://www.net54baseball.com/showpos...7&postcount=41
Roller transfers produce clear ghosts. I am not a printing expert, but I believe this happens when ink is transfered to the roller that is used to feed the paper through the press (probably because the press was run without a piece of paper in it). When the next sheet of paper is then sent through the press it prints the layer it should on the front of the card while the roller that helps move the paper through the press transfers the other image (in reverse) to the back. These are great images:
http://www.net54baseball.com/showpos...2&postcount=11
Scraps/test prints are cards where the printer intentionally printed over another image as a test run and then scrapped it. These ghosts can be either on the front (Johnny's Bender-Lake, posted here because I know he won't mind that I shared it) or the back, like the Mullaney that was formerly Leon's:
http://sports.ha.com/itm/baseball/-t...umbnail-071515