Stupidity: The lead Predator got infected with a Xeno chestburster, and at the end of the movie, his body is shown on the ship with it bursting out. This is what sets up the sequel. However, the Predators have been coming to Earth for centuries, and they have also been hunting the Xenos for centuries and know their behaviours and breeding habits. They have X-ray capability and even use it in this film to show a human is infected. Yet they never scan his body before bringing it on their ship. And yes, I know there's the whole thing about them coming out victorious, it seems, and the other Predators likely would assume they were untouched. That doesn't make it not stupid that they still didn't scan his body to be sure, as they know what these things are capable of. And of course, in the sequel, that stupidity bites them in the ass.
Alien Vs. Predator (2004)
1 stupidity
Directed by: Paul W.S. Anderson
Starring: Lance Henriksen, Ewen Bremner, Sanaa Lathan, Colin Salmon, Raoul Bova
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)
Alexa 'Lex' Woods: How do you say "scared shitless" in Italian?
Sebastian de Rosa: Non vedo l'ora di uscire da questo piramide con te, perché mi sto cagando addosso. More or less.
Question: Can anyone tell me why the Predators didn't scan the 'token' human survivor or the last dead predator to see if either were infected with an Alien? Especially when the Predator was not wearing his face mask which would serve as protection against this infection.
Chosen answer: Given the speed with which the chestbursters emerged from the other humans, it seems likely that these have been engineered by the Predators to emerge as quickly as possible. As such, they probably assumed in Lex's case that, were she impregnated, it would have emerged before that point. As for their fallen comrade, it may well have simply not occurred to them that one of their number would be foolish enough to get himself impregnated. Even proud alien hunters can make mistakes.
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.