Continuity mistake: When the man is pinned to the wall by the Predator's spear there is a close-up shot of his feet showing a box beside his right foot. When it cuts to a shot from the side of the man the box is gone. (00:31:35)
Continuity mistake: Towards the beginning, in the scene where they are first going down the tunnel, and the one guy slides out of control. When the woman uses her pick axe to stop him, she slams it into the tunnel before he passes, and his jacket catches on the ax. In the next shot, the axe has struck the jacket in the middle, the jacket is no longer "hooked" on the ax.
Continuity mistake: When the alien queen awakes she opens her mouth and screams. When she does so she lifts up her upper lip exposing her teeth. When it cuts she does the same again. (00:29:40)
Continuity mistake: When Sebastian says "Thomas, the burial chamber is here. We're gonna find it." you can see a man with a blue shirt and a man with an orange shirt walking in the background. When it cuts the same men are walking but now several people have suddenly appeared behind them. (00:05:30)
Continuity mistake: In the scene with the sacrificial chamber, where the Alien eggs come up and start to open, the guy with the glasses (if someone can correct me on the name, please do), he shines the flashlight on the egg closest to him and focuses the beam tighter. As the camera angle shifts to a higher place, he does the same thing again. (00:41:05)
Factual error: Lex reaches the top of Lho La Ice Fall (Nepal) to find Max there standing in front of an Eurocopter AS350 helicopter. The problem is that this helicopter has a service ceiling of about 15,000 ft., and Lho La Ice Fall is at almost 20,000 ft. This is far beyond the capabilities of the helicopter's performance. (00:03:18)
Continuity mistake: In the beginning when the man is running through the whaling station the two upper buttons on his shirt are unbuttoned. But when he falls on his back as the Predator is approaching him his shirt is entirely buttoned (Extended version only). (00:01:20)
Continuity mistake: Below the sacrificial chamber when Lex says "This should be below the sacrificial chamber" you can see Lex and Sebastian walking towards the stairs, while Maxwell and Thomas are several feet behind them. When it cuts to a shot from above them they are all suddenly clustered together. (00:36:55)
Revealing mistake: When the second man is killed by the Predator and thrown through the air hitting a table, if you look closely the feet of the table have been sawn in advance, making the table break easier. (00:31:15)
Continuity mistake: When Weyland drops the Predator weapon we see from the Predator's view that Weyland grabs the weapon at the end of it. When it cuts Weyland's grip has moved further in on the weapon. (00:50:50)
Continuity mistake: When Lex and the Predator are running through the tunnel and neither of them has the spear that the Predator built for her. Then later spear is back. Then after the sled crashes, its gone again. For the remainder of the sequence, the Predator has it. (01:19:15 - 01:20:40)
Continuity mistake: In the beginning when the man realizes the Predator is in the same room, he falls to the ground and backs up against an oven. You can see several dippers behind his back, but only three dippers on his right side. When it cuts to a close-up there are five dippers. (Extended version only). (00:01:25)
Continuity mistake: As Quinn is sliding down the tunnel while trying to grab on to the ice with his icepick, he is sliding down with his feet first. Then he is suddenly sliding backwards with no time to have rotated. (00:32:35)
Continuity mistake: After Sebastian manages to open the sarcophagus containing the weapons, Miller sticks his head in to the sarcophagus and says "Any ideas what these are?" When it cuts his head is out of the sarcophagus. (00:39:00)
Continuity mistake: When Lex comes climbing over the edge she rams one of her icepicks in the ground. Two shots later she has suddenly rammed her other icepick in the ground as well. We would have noticed if she did so. (00:04:25)
Continuity mistake: Lex gets the Predators glowing green blood splashed on her clothes when the alien kills it. Next shot her clothes are clean.
Answer: When they emerged from the underground pyramid unscathed, the Predators assumed they were not infected. Any Predator, young or old, would have been taught to self destruct rather than be caught or killed. As seen in the previous movies. Lex was given the mark of a warrior, they assumed she was taught their code of honor.