Freezing and thawing living(or once-living) tissues is a difficult business. Ice crystals get into certain parts of the body and when you heat that sucker up, you've got ice melting and tissues reheating, so there's bound to be some gooshy stuff going on.

I think it depends on mouse and snake. Sometimes Cap gets just a bit too excited with dinner and he ends up squeezing the poor dead mouse like, well, a sponge. Other times he eats cleanly and calmly. I'd be concerned if it was bleeding before being thawed, even if I knew that it had been frozen and thawed well.