Revealing mistake: When Jason (in Robert's body) is shot in the head at the end of the hall, a close up reveals the bullet hole to be obviously just makeup. (01:05:35)
Revealing mistake: If you watch closely, it is painfully obvious that in the beginning, when Jason is getting shot up, they are just repeating the same shot from different angles.
Revealing mistake: When the female FBI agent falls over the railing onto the table, you can see that the legs have been pre-bent to make it break easier. (00:04:50)
Revealing mistake: In the scene where Deborah and Luke are having sex in the tent, the coroner (as Jason) comes from behind her, stabs her, and brings the poker upward slicing her upper torso in half. If you look closely in normal play, or put in slow motion, you can see the body is already cut before the poker comes up. (00:32:00)
Revealing mistake: Jessica is in the shower crying, and then she grabs a shirt and pulls it on, never putting on anything else. However after Steven punches Robert, he carries Jessica to his car. As he puts her down on the seat, Jessica's shirt rides up, revealing a flesh-coloured thong. (00:58:45 - 01:00:40)
Revealing mistake: When Edna's head is slammed in the door, it is completely bloodless and her head appears to bounce back to normal shape. (00:34:55)
Revealing mistake: Jason is being bombarded with gunshots and the machete isn't damaged at all. The bullets magically bounce right off it. And "spark".
Revealing mistake: At the end of the film when Freddy's arm bursts out of the ground and grabs Jason's hockey mask, you can see none of his skin is burnt, like it should be, and also they metal blades on his glove clearly bend like plastic. (01:23:50)
Revealing mistake: After Steven cuts possessed Randy's throat with a machete, you can see that the machete is completely free of blood. (01:15:55)
Revealing mistake: Phil, who is possessed by Jason, puts shaving cream on Josh's moustache. Then, he takes a razor and begins to shave his stubble off, and then his moustache - however, you can quite clearly see that Josh's moustache is not there anyway, and has disappeared between shots. (00:36:00)
Revealing mistake: As Agent Marcus runs through the woods, away from Jason, all she wears is a white towel around her body, which is just tucked in and over at the top. However it is very obvious that the towel is clipped together round the waist, in order for it not to fall of, most obvious because she trips, climbs over branches, etc. and the towel is always tightly secured. (00:05:15)
Revealing mistake: After Vicki has been killed, Jessica mourns by her body. She then proceeds to stroke Vicki's hair and face. Well, Vicki dies with her eyes wide open. However, as Jessica strokes her face, Vicki closes her eyes. Her eyes then remain closed for all the following shots of her. (01:10:20)
Answer: There isn't much of a feasible in-movie explanation. In truth, the entire scene was added by the director because he felt the movies were pretty sexist in that it was mostly women who were either nude or sexualized. Thus, he added an inherently homo-erotic sequence involving one man shaving another stripped man to "level the playing field" so-to-speak. The only real explanation I could think of is that maybe the creature just thought it'd feel uncomfortable transferring to a man with a mustache. But even that's shaky at best.