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.
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.