Yes, if the rat was exposed to any toxins or if the rat has started to decompose too much. That's why we don't feed wild caught rats/mice etc and always do the "sniff test" before offering once you have defrosted the prey if using F/T.
I also advise not re-freezing F/T if the snake doesn't seem interested - the next time you defrost it it will decay even further during thawing and eventually you end up with a "rat grenade" that busts open when the snake strikes. Not a great deal of fun for anyone that.