I noticed this sometimes happens with fire damage done to pandorans will receive negative 2 billion damage (possibly other enemy types as well, I don't know). I don't remember the exact number, it only flashes on the screen for a second but it was 2 "something". I also dimly remember that the greatest magnitude number that can be represented by a 32 bit integer is also 2 billion something. I don't know if these two things are related or not, just thought I would pass it on.