This is probably the least significant bug in the game, but I've figured out how to get illegal numbers for players. You can reassign a player to any position where that number is legal, then change his number to that number. For instance, I've made this QB #57:
http://mfn1.myfootballnow.com/player/7361Additionally, I've been able to give a player #0 as well. This was totally an accident, but I signed a LB when all of the numbers had already been taken, and he's been auto-assigned #0:
http://cust10.myfootballnow.com/player/230I understand there are much bigger issues going on elsewhere in the game, but a bug is a bug.