Since when can a unit have negative HP? Did I miss something?

I know if you put a very high number in the HP Bar of a unit it overloads and resorts to negatives, Like If I was to put 10,000,000 or something, it's end up a massive negative number.

Units with negative HP are one hit by any amount of damage I beleive.

Yeah, I put a lot of nines for testing purposes, and got a negative. Alright, thanks then.