Revealing mistake: There is a wide shot of the ship were there is some equipment on the ice and some people moving around. The ship itself and the equipment cast shadows. But none of the moving people cast any shadows at all. (00:14:25)
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 places his camera on the cabinet to take a picture of himself, the string from the camera changes position between the shots. (00:19:50)
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)
Factual error: When they're first entering the whaling camp, a penguin startles Miller. That penguin was a black-footed penguin, which is a bird native to much warmer climates. If that little guy had actually been in Antarctica, he would have died within minutes. (00:20:40)
Revealing mistake: In the beginning of the film, the penguin walks on artificial (paper) snow. (00:20:40)
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)
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 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: 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: Close to the beginning, when the group is inside the pyramid below the sacrificial chamber, when the shot is from behind Lex and the others, note where the green light is, directly in the middle of the stairs. When it cuts to an overhead view, it's more to the left than the right. (00:36:50)
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 Lex and the others in the group reach the sarcophagus, Sebastian starts to turn one of the dials, the camera cuts to Weyland for his line, he has a flashlight in his right hand, yet in the following shot its no longer there. (00:37:50)
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 the group are in the pyramid, just after they have discovered the weapons inside the sarcophagus, they begin to take them out. When Sebastian says, "Don't touch them", all the doors start to close, and Lex runs past a guy twice. (Slo-mo may be necessary.) (00:40:15)
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: When the group walks along a passageway, the predators are watching from above, and in one shot the distance of the guy at the back of the group goes from about 8 feet, to almost right next to the others between shots. (00:44:55)
Continuity mistake: When the predators are watching the humans in a room, Weyland drops one of the guns from his backpack. Its position and distance on the floor from Weyland's boots changes between shots. (00:50:45)
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 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)
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.