How much frozen? If its a just a little on the inside you're fine. Just make sure the majority of the rat is de-thawed properly. I use hot water exclusively to de-thaw my rodents and it works just great. My boas take f/t that have been about 90% thawed and never once had an issue or a regurgitation. You don't want to shock their system with a frozen rodent. Frozen stiff limbs can harm the snakes internal organs. But again, if its just a little on the interior of the rodent don't sweat it.