The valves will burn. If driven long enough the pistons will seize.
My brothers 2011 seized up on me while spreading manure. On the end of the asphalt road, where it turns into a dirt road, it seized. I checked the water and found nothing in it. After 5 minutes of cooling it started again. i let it idle while filling small amounts of water from a ditch, starting with just some splashes, so i wouldnt crack the heads due to sudden cooling. Topped it up and got back to work. The engine did not seem to have suffered any damage from it, however it already had 40 years of wear on it.