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In Part IV, Trish tries to escape from a house, but finds several of her friends' bodies nailed across the doorways. Cue screaming. However, one has to imagine the scene of Jason taking the time to find a hammer and nails and propping them up just for the pure sake of freaking out someone he was just going to kill anyway.
There's also Ralph hiding in the pantry in the original film; you have to wonder how long he was in there before Alice found him.
In Part VIII, apparently Rennie had been dragged down by Kid Jason, leading to her hydrophobia. It couldn't have taken place any later than perhaps late 70s or early 80s. With that said, Jason himself "drowned" in '57, leading to his mother's own rampage the next year and in '80. So either Jason got split into the lake dwelling Kid Jason and his adult version we all know and love, or Rennie's near drowning was so traumatic that she justified it by imagining it was Jason who had nearly drowned her. Also, Kid Jason has hair, though that may be more of They Just Didn't Care.
Jason X. Jason is wanted for his "regenerative properties" - but they already have nanotechnology that can heal anyone instantly of any wound. Even Jason himself. Guh?
He was wanted for his regenerative properties by people in the 2011, not by the people in 2455. He was revived in the future because of his potential value as a nearly 450-year-old living relic.
Replicating his instantaneous healing factor could still be useful, since you wouldn't always have access to the nanotechnology.
And the starship explosion. The remaining crew decide to explode the walkways leading to the section where Jason is, so he will be forever floating in the vacuum of space. So they do. And it works...mostly. Jason is now apparently Made of Indestructium due to the nanobots...and can also move freely around space, because he comes right back about a minute later, punches a hole in the spaceship hull and causes Jenessa's Explosive Decompression.
In Part I we see two counselors having sex in one of the cabins. Afterwards, we see the guy laying down, and an arrow is pushed through his neck from under the bed. OK, but then you realize, that means the killer was under the bed the entire time they were having sex! Eeeeeew.
Whenever Jason talks to the "ghost" of his mother, there's a very good chance these are hallucinations. Pamela probably never wanted her son to become a killer.
Finding out Jason was never dead to begin with in the sequels. This means everyone in the first film (including Pamela herself) died for absolutely nothing.
Why doesn't Jason try to kill children in the later sequels? Because last time he tried it, he died for real.
The first film opens with the POV of the killer as she follows two councillors into the attic. Before proceeding with the murder, the camera pauses over some sleeping children, before moving on without harming them. This is foreshadowing the killer's identity as a grieving mother. It's likely looking at those kids reminded Pamela of what her son was like in life.
In Jason X, Adrienne, the blonde who runs tests on Jason's body, removes his mask to get a look at his face (presumably at least somewhat out of curiosity). When Jason awakens, he probably realizes that someone has seen his face, and if we've learned anything throughout the series, it's this— Jason takes two things very seriously: his mother, and his looks. At various times during the franchise, he shows something along the lines of self-consciousness towards his disfigurement.
So what does Jason do when he's pissed off at someone for looking at his hideous, rotting mug beneath the hockey mask? He returns the favor. He dips Adrienne's head in liquid nitrogen, freezing it nearly instantaneously, and smashes her face against the counter next to the sink. Jason, still holding her body by the back of the head, actually turns (what's left of) her face toward him and looks at his handiwork (a concave slab of featureless frozen skin and blood). Jason's no dummy— he knows when people are dead, and generally, even if they aren't, he doesn't care and just assumes they are. This is the giveaway that Jason was sending a message. "Oh, you think I'm ugly? You're about to look even worse."
The ship being named Grendel in Jason X is exceptionally fitting. Both Jason and Grendel were monsters who dwelled in a lake, both came out and went on murderous rampages when people nearby were engaging in festivals and debauchery, and both had a strong devotion to their mothers. You could even go so far as to say that Jason Voorhees is the modern equivalent of Grendel.