For what it's worth, I agree with those who feel that, if Disney really cared about this issue, they would find some way to enforce it. The mugs do say that they are good for free refills at the resort at which they were purchased for the length of that particular stay. But since Disney has chosen to make all the mugs identical, and since they have failed to implement any method to determine when the mugs were purchased, the logical conclusion would be that they aren't especially concerned about this. Yes, it remains a technical foul and is against the rules, but this seems to be one of those rules honored more in the breach than the observance, apparently with Disney's tacit approval.