Continuity mistake: In the beginning when Lex answers the phone, her earpiece and ear are visible. In the next shot both are covered by her cap. (00:03:55)
Continuity mistake: When Alexa and Sebastian are walking down a passageway in the pyramid, after everyone else has been killed, they begin to hear aliens all around them and look around. When Alexa shines her torch at an alien, she is on Sebastian's right side, but in the next shot she is on his left side, and her small torch vanishes from her right hand. (01:05:10)
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)
Factual error: The Desert Eagle that Miller & Lex use to shoot Aliens can hold a maximum of 9 rounds in .357 Magnum. Yet they fire quite a large number of shots from the same gun without reloading. High-capacity magazines are unavailable for the Desert Eagle as the magnum rounds are just too big. (01:02:45 - 01:14:00)
Continuity mistake: When Sebastian has picked up the Pepsi Cola cap and about to uncover it, as Thomas is filming him, we can see his sleeve on his right arm is pulled up. When it cuts to a view through Thomas' camera his sleeve is further down his wrist. (00:05:15)
Factual error: The members of the expedition are wearing their Snow Goggles on their hats with the lenses to the front. However, anyone who has ever worked in a cold weather environment (especially the guide and her "7 seasons on the ice") would know that the goggles should be worn backwards if they're not over your eyes. This keeps them from getting moisture on them.
Factual error: The movie takes place in the first half of October which is the beginning of the "Antarctic summer" not that far away from the Antarctic circle. That means there should be a lot of sunlight and sun should only set for couple of hours. However all the exterior scenes in Antarctica are shot in darkness.
Continuity mistake: In the beginning, when the man notices the "Unidentified Heat Signature" warning on the screen, he removes his arm from the table twice in separate shots. (00:03:05)
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: 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)
Factual error: When the predator starts to kill Weyland, Weyland uses his oxygen tank as a makeshift flame thrower. Contrary to popular belief, oxygen does NOT burn. It only allows other things to burn. The three things needed for a fire are oxygen (which he had), fuel (which he did NOT have) and heat (from the flare he lit). (00:59:25)
Revealing mistake: In the beginning of the film, the penguin walks on artificial (paper) snow. (00:20:40)
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)
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.