A long time ago, Blizzard was hired by Games Workshop to make a WHFB (or WH40k, i forget) game. Something happened, and GW didn't want the game after a while, so Blizzard pretty much went ahead and changed some of the names, and called it Warcraft (or Starcraft). It was wildly successful and GW has been butthurt over the incident pretty much to this day.
Or something like this, I'm honestly not sure.
EDIT: Upon doing some brief research, it was both Warcraft and Starcraft, and the falling out was supposedly because GW was a turd (what's new?).