Continuity mistake: In the scene where Jessica is in the shower crying and the power goes out, she gets out, puts on a white t-shirt and goes walking around the house. When she walks through the garage door and grabs a hammer, there is blood on the front of the t-shirt (right at or a little lower than her waist and looks like someone grabbed the shirt with a bloody hand). Where did it come from? She didn't run into anything or anyone. [There's a deleted scene showing her step on a piece of glass. She gets blood on her hands when she pulls the glass out, then presumably wipes her hands on her shirt. Still a mistake, as removing that scene means the blood appears from nowhere, but there's the reason.]
Continuity mistake: When the bomb is dropped, Jason is looking skyward, but in the next shot he's looking straight forward.
Continuity mistake: There are two times after the female FBI agent gets out of the shower that she is wearing underwear when she put none on, and the pair that she has on is completely different from the one she originally took off. The first is when she runs directly to the door from the shower after the power goes out. The second is seen when she puts on only a towel shortly after, and then falls over the railing, you can see she's wearing underwear there as well. (00:04:45)
Continuity mistake: When the coroner is inspecting Jason's body he is wearing a watch on his left wrist. While he is eating the heart the watch disappears but returns after he is possessed. (00:13:35)
Continuity mistake: At the start of the film Jason chases Agent Marcus from the cabin, and she is barefoot. But in the shots of her running through the woods, in some shots she is barefoot, in others she has white socks and big black boots on, and in others she has flesh-coloured trainers on. (00:05:10)
Continuity mistake: When Steven impales Josh with the fireplace poker, Josh starts to stumble very slowly toward the window, but in the next shot, he is moving with enough force to move across the room and shatter through a window. (00:39:25)
Continuity mistake: When Jason is attacking Steven in Diana's house, before Steve impales Josh (who is possessed by Jason) with the the poker, Josh lifts his head to look into the mirror but the reflection of Jason stays still. (00:39:15)
Continuity mistake: The position of Edna's headscarf changes between shots before her head is slammed in the car door. (00:34:55)
Continuity mistake: After Jessica shoots Josh in the head, he lies on the floor and his tee-shirt is clearly untucked. But when he grabs Jessica, and then wrestle on the floor, his shirt is now tucked in. (00:38:00)
Continuity mistake: In one inside shot of the tent, Luke is lying down on his chest and he turns the lantern on. In the very next shot from outside, it shows his shadow cast from the inside of the tent, and he is now sitting up. (00:26:30)
Continuity mistake: When Agent Marcus is replacing the burnt bulb while standing on the ladder, between shots the bill of her baseball cap folds itself upward. (00:03:00)
Continuity mistake: On the field where Jason is being shot, Jason is equipped with a machete in his right hand. However when they drop the bomb onto him, Jason hears it and looks up, and in this shot you can see that Jason's machete has disappeared. (00:06:35)
Continuity mistake: When Luke and Deborah go into the tent, their shadows show Luke automatically take off the shirt she is wearing. But the next shot, when the camera goes into the tent, Deborah is once again wearing the shirt.
Continuity mistake: When Agent Marcus arrives at the Crystal Lake cabin at the start, the sun is low, but the sky is still bright. In the next shot as she goes into the house, the sky is pitch black. (00:01:10)
Continuity mistake: The note that Jessica found in Stephanie's crib changes. When she finds it, it reads "I have what you want " and is written in blue ballpoint pen. But when Steven finds the note later, it reads "I have your baby " and is now written in thick, black permanent marker. (01:08:35 - 01:11:15)
Continuity mistake: When Deborah is impaled with a post in the tent, the amount of blood on Luke's face decreases between shots. (00:32:00)
Continuity mistake: After the female FBI agent falls over the railing and through the coffee table, the two legs closest to the screen roll away, but when the table is shown again, the legs are underneath it. (00:04:50)
Continuity mistake: The father of Jessica's baby, while in jail, had some of his fingers broken by Craitan Duke, but in the next scene, he is fighting another character and his hand with broken fingers is working fine.
Continuity mistake: When Steven gets punched by Randy there is blood filling his mouth, but a few seconds later there is barely any blood on his lips. (01:03:25)
Continuity mistake: When Steven is fighting with Randy by the police car, you can clearly see in one shot that he has his gun in the back of his pants, but later he pulls the gun out of the front of his jeans. (01:03:15)
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.