Stupidity: When Anna, Velkan and the villagers successfully trap the gray werewolf in a huge cage, the villagers start firing on it. Anna says they need to use Velkan's gun because it has silver bullets in it. Since everybody knew they were fighting a werewolf, all of them should have had silver bullets for their guns.
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.