Yeah, that usually happens when I've over-warmed the f/t rodents. Also nasty: doing a warmed rodent puppet show with a pair of hemastats, only to have the skin & fur peel off.
I've learned to hold the rodents in my hand for few seconds if I suspect I've overheated them under a lamp. If they feel like they're too hot inside, then I don't really want my snakes swallowing them. So I throw them out and warm up new ones.