I can't see this as an honest mistake. He had already shot one buck, used a whitetail license on this buck over a month after the mule deer season ended. He then knowingly hauled the buck across state lines to get it mounted, then put it up at a sportsman's expo. That shows he had no regrets for it. People do make mistakes, but people do not glowingly show their mistakes off.
I would forgive if it seemed to be an honest mistake, like someone trying to put some meat in the freezer with the wrong species of deer. But not when it is a trophy that many attempts had been made to hide the instance.