Have I Heard That Song Before?
Whenever any of the characters plays a radio, any time during the film, its always the same song being played.
Did Jason Drown During the Day or Night?
When Jason's mom, Mrs. Vorhees, is telling Alice of her son's drowning, she says that two of the camp counselors were making love while he died. At the beginning, the counselors are making love at night. When Mrs. Vorhees "sees" Jason at the end, he is drowning during the day.
Who Built That Fire?
When Mrs.Voorhees is telling Alice about Jason's drowning (at the end of the film), if you look out the big window behind them, you can see a camp fire burning not that far from the cabin. Who was left alive to build this fire? They just had a heavy rain storm!