I came across the miniboss "aspect of destruction" in act 4 with -2^31 health - that's −2,147,483,648... we fought it for a looooooong time before noticing that it had 1 health and was not dying.
Interestingly enough I came across this same group of minions with an extra life mod on them. The final one that we killed had a bar that said 400 Million hp. You heard that right, 400 million. We fought it for like 30 minutes out of spite. He dropped a radiant emerald.