Revealing mistake: Mac and Dillon go after the Predator. The Predator is sitting in a tree, and Mac points him out to Dillon. They both agree that they see it and take position. Mac crawls underneath the alien and is killed by his laser. The Predator then sees Dillon and lasers his arm off. Dillon's arm falls to the ground with the gun firing. Quickly Dillon turns and reaches for the machine gun around his neck. When he turns you clearly see his bloody right stump and his right arm sticking out the bottom of his jacket. [Note: As far as I can work out, this has been cunningly edited out of the UK DVD version].
Revealing mistake: In the scene where Dutch is being chased by the Predator, near the end, he jumps into a waterfall. When you see him take a high-dive, it is archive footage of a different person, filmed on a different camera. When it cuts straight back to Arnie, you can see that it was a completely different place as well. When you know to look for it, this is really obvious. (01:17:05)
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.
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.