Revealing mistake: After they catch the Predator in the net and it breaks free, the laser blast breaks a big part of a tree and it heads toward a soldier, hitting him head on. If you watch closely when they show an above shot of the tree hitting the guy, you can see an air pressured pad pushing the tree on the ground into him. (01:06:40)
Revealing mistake: When Arnold is heading into the trees after building his bow and arrows you can see a silver eye bolt on the top of the bow where the string would attach.
Visible crew/equipment: When Arnie says 'oh s***' and falls off the cliff, watch as the camera goes over the edge. If you look on the mountain, you can see a shadow of a black ball with a cable attached: this is the camera filming the shot which has been thrown off the cliff.
Revealing mistake: When Bill Duke picks the scorpion off Carl Weathers' shoulder with a knife, you can blatantly see some gum or similar that's been used to stick the scorpion onto the tip of the knife. Even if it's meant to be Bill Duke's gum, it's not on the tip of the knife before he stabs it.
Visible crew/equipment: Watch when Arnie is sliding down the cliff: it shows a shot from his eyes looking down at his feet, and you can see the wooden platform he's sliding down.
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)Phoenix
Continuity mistake: During the montage scene of the remaining men setting the net trap for the Predator, all of the men are each doing individual jobs. Dillon does not take part initially, until Arnold says 'Instead of complaining, maybe you should help.' However, long before Arnold tells Dillon to help, Dillon can actually be seen helping out already. Watch closely...when Mac throws a rope across to Billy, Dillon is standing behind Mac, with his shirt off, fastening a rope to a tree, as if he was helping the whole time already. A similar shot occurs after Arnold's comment, but the first one is definitely before. (01:02:05)
Continuity mistake: When Dutch is running from the Predator after he sets the grenade off in the tunnel sequence, he avoids the Predator's gun blast by leaping and swinging on the branch of a dead tree. As Dutch's flying through the air, you can clearly see that it is a real branch. But as the screen switches to an explosion and back to Dutch, the branch is now some prop 2x4 and collapses as Dutch now plunges into the pond below. (01:33:20)