Other mistake: When the Predator sees Billy on the log bridge, there is a cold blotch in the middle of Billy's chest. He's not wearing anything that would make such a hole in his heat signature - it should be the same color as the rest of his torso. It doesn't even match up with the cut he made across his chest; the cut starts much higher than the blotch and the blotch is irregularly shaped, not indicating the smooth line of a cut. (01:15:20)
Other mistake: Right at the end, Arnold fires an arrow at the Predator which barely takes flight and fires very slowly. He is shown making the bow very tight so the arrow should come out very quickly.