The 1989 Upper Deck Gary Pettis is pretty famous, because he is shown holding the back of that card.
What is interesting, however, is that if you look closely there are a lot of differences between the card back that Pettis is holding and the actual card back.
The top left with his name and vital information is not there, and there is no background behind the stats. Also, the picture cuts off at a different place on the left side. Not all of his leg is visible on the prototype card, but the whole leg is visible on the actual card.