Plot hole: After Blaine gets killed and the group hose down the forest, Poncho goes and checks for the enemy. He comes back and says he didn't find tracks or blood "We hit NOTHINGGG." Now, while he would be looking for red blood, he certainly should have noticed at least the glowing Predator blood (made from glow stick fluid - widely used by the military and public alike at the time) easily visible on the large leaf, and deduced that came from something - perhaps the enemy soldier had a glow stick.
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].
Factual error: The predator enters the compound and picks up the scorpion. The silhouette of the scorpion changes from bright to dark, showing a drop in temperature. But scorpions are cold blooded animals who have an ambient temperature. The predator should never have seen it at all.
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)
Factual error: The heat from Dutch's body would warm the mud up in a matter of seconds (tested by the Mythbusters). It doesn't even cover all of his skin or his eyes. And the Predator isn't filtering his input for only large heat sources. His normal vision tracks in on a rat hiding just above Dutch.
Factual error: Ventura carried the M134 Minigun and an ammo pack which holds 500 rounds. At its firing rate, that ammo would be spent within a matter of seconds, not minutes as was seen in the movie. (00:47:45)
Revealing mistake: When the Predator self-destructs and the people in the chopper look out at the blast, the "painting" of the explosion is still and does not move or change like a normal explosion would. (01:41:05)
Continuity mistake: When the Predator blasts free of the net and his laser blows off a tree limb; in the first shot you can see the limb is hollow as it is shot off [and I for one have never heard of a hollow tree, but even if they do exist...] as the free limb swings down toward Poncho, you can see that it is no longer hollow, but completely solid. (01:06:40)
Continuity mistake: As Dutch climbs out of the water he crawls through mud. He looks back at the pursuing Predator and crawls backward through the mud. Even with all this crawling there is no way that he would have been covered head to toe in mud. If you look closely every little nook and cranny is full of mud, he would have had to dunk his entire body in a mud pit to accomplish this. (01:19:25)
Continuity mistake: In the scene where the female prisoner falls to the ground and pretends to be hurt, then throws leaves in the soldier's face and tries to escape. She is stopped by another soldier, standing in her path with a gun in her face. The sequence with the gun pointed at her head is filmed in two shots. One looking up at the soldier holding the gun, the other with the gun hovering in her face. Every time it changes between these two camera angles, we find the soldier's hands in two completely different positions on the gun he is holding.
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.
Factual error: In the scene where Dutch is making his improvised explosive arrows we get a close up of him dismantling a 40 mm High explosive grenade and dumping the propellant out onto a large leaf. Way to much powder came out. An actual 40 mm has only about an amount equal to a sewing thimble full.