When a policeman shows up at the camp looking for Crazy Ralph, it is badly timed to Ned running out of the cabin dressed as an Indian, screaming and dancing around. He says "oh shit" when he spots the cop but the characters don't bat an eyelid. It adds nothing to the plot and is just so random.
There's a conversation between Jack and Marcy that has nothing to do with anything really and has no impact on the plot. Just before going into the bunks to do it, they have a talk by the lake. Marcy tells Jack about a dream she frequently has where it's raining and the rain turns to blood.
Right before she's killed, Marcy is in the outhouse and randomly looks into the mirror to do a Katharine Hepburn impression, quoting The Rainmaker.
Critical Dissonance: Critics more or less hated it, but it went on to be one of the biggest slasher films ever made.
Epileptic Trees: Before Brenda dies, she's lured to her fate by the sound of a child calling for help. When we see "Jason" talking through Pamela's voice, Pamela just uses a slightly higher version of her voice, nothing like the child. Another thing to note is that when Brenda's body gets thrown through the window, it would have been difficult for Pamela to run to her Jeep from that distance from the cabin right before she meets Alice. When Pamela sees Brenda's body, she apparently feigns innocence by wondering who did that to her before immediately dropping the act to chase Alice. This has led some to theorize that it was actually Jason that killed Brenda rather than Pamela.
It Was His Sled: The killer is probably known to most people who are familiar with the series. Though to those unfamiliar with the series, they still might be surprised to see Jason's lack of appearance. This makes the twist at the end all the more effective 30 years later for those coming in the first time expecting Jason to be the killer.
Jerkass Woobie: Pamela Voorhees. The novelization implies she only went on a killing spree to save further campers from Jason's fate.
Narm Charm: Mrs. Voorhees alternating her voice between her own and "Jason's".
Tear Jerker: The villain isn't Jason, but rather his mother Pamela, who went completely insane after the supposed "death" of her son. Then the ending and the sequels reveal he was never dead to begin with, meaning her horrible deeds accomplished nothing — except to inspire an innocent child to become a violent serial killer in her name.
This troper actually has a theory that Pamela was using a blood sacrifice to bring Jason back.
Unbuilt Trope: Alice is the film's Final Girl, yet you'd be surprised that she a) is implied to be having an affair with Steve, b) provides Fanservice in the form of a bikini, c) happily smokes and drinks with the others, and d) is about to strip during the Strip Monopoly game (but gets stopped when the door blows open). She's also one of the few Final Girls to be blonde.