One of the members of the TCDB recently discovered that the 1991 Ultra set had a previously undiscovered variation. The NFLPA logo, which contains a football, shown on the reverse of the cards, comes in two sizes. The front of the card is the same in both cases. Here is the front of Herschel Walker's card:
Now here is the reverse with the larger logo, which I recently added to the Hoard:
Notice the size of the football in the logo in the upper left. Now here is the other version, showing the smaller size football:
As can be seen, the football in the second example's logo is much smaller. Though not confirmed, it appears that each card in the 300 card set comes in both sizes.