Continuity mistake: In the beginning when Sebastian is uncovering the tunnel he says "We must be right on top of it", you can see he rests his right arm over the edge of the bricks. When it cuts from inside the tunnel his arm no longer over the bricks. (00:04:45)
Continuity mistake: In the close-up shot inside the snowmobile, where we see Sebastian fiddle with his bottle cap, we can see he is holding it with his right hand. When it cuts he is holding it with his left hand. (00:16:00)
Continuity mistake: When Miller and Lex see the penguin it travels some distance from the table it emerged from, yet in the following shot, it is a lot closer to the table. (00:20:35)
Continuity mistake: In the scene when the Predator is impaled on the Alien's tail, there is Predator blood all over the Alien's tail. When the Alien flings the Predator's body away, the Predator blood on the Alien's tail disappears. (00:52:30)
Continuity mistake: At the end compare the shots taken from the top when the predator is lying dead over the snow; right after he dies and when the other predators are going to pick him up. His left arm changes position and his "rasta locks" are spread in a different pattern. (01:23:40)
Continuity mistake: When Sebastian starts to crawl back out of the hole after discovering the Pepsi-cola cap, the guy with the camera is holding the camera with his right hand, but in the following shot he is holding it with both hands. (00:03:50)
Continuity mistake: When the predator lifts his spear before killing Max, the predator holds the spear by his chin level. When it cuts, the spear is almost over the predator's head. (00:51:40)
Continuity mistake: When the main Predator kills his first alien, he concludes the 'rite of passage' by burning himself on the forehead with alien acid. This should have left a circular mark on his forehead, but when we see him at the end of the movie, he only has the 'tau' symbol. (01:00:35 - 01:20:40)
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)
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)
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)
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.