Back when we photographed with film, my brother once took a picture of the Astrodome after a baseball game, and then a picture of Joe Morgan - the film didn't advance, so he had a more faint 'ghost' image of Morgan over the vivid Astrodome image, and the result was very cool-looking.
With cards, I considered a ghost to be remnants of a player's image somewhere else on the card, generally off to the side by a good amount. Some are more dramatic than others, but the ones like the Tenney, to me just seem to be bad registration - interesting, but personally of no interest. On printer scrap, as Johnny said - it could be a completely different ball-player. It might look 'ghostly', or it might look like junk, but it's got big $$$ potential.
The red in this Schlei, if just off by 1/16", would simply look like horrible registration; however, pushed this far to the side, really does look like a ghost hanging over his shoulder:
