Acting in the Dark: Adrienne King didn't know Alice would be killed off and there was no script for her scene. So she showed up and ad-libbed a conversation on the phone with Alice's mother.
Billing Displacement: Adrienne King gets a third billing, even though she has less than 10 minutes of screen-time and most of that is just a flashback to the first film. The credits don't even begin until after she's killed by Jason.
Crosscast Role: Jason's legs in the beginning of the movie is that of a woman, the only time in the franchise Jason was played by a woman.
Dawson Casting: Averted with Marta Kober, who played Sanda. Her nude scenes had to be cut down when it was discovered she was actually underage.
Disowned Adaptation: Betsy Palmer, Tom Savini, Steve Miner, and Victor Miller all felt it was a dumb idea to have Jason turn out to be alive the whole time, leading to the Fridge Logic of why he didn't say anything to his mother. The first three eventually warmed up to the idea, however, especially Palmer.
Dueling Works: This came out the same year as Halloween II (1981), the second entry in the franchise that Friday the 13th was created to emulate. Both films also feature the killer murdering a cop with a hammer to the head.
Production Posse: Most of the original crew from the first film returned to work on the sequel.
Laura Marie Taylor (Vicki) had a legitimate crush on Tom McBride (Mark), lending realism to Vicki flirting with Mark. Unfortunately for her, he was gay in real life.
As noted above, Adrienne King was dealing with a stalker at the time. Knowing this makes Alice's death scene, in which she's killed in her own home by Jason, that much more disturbing.
Shrug of God: How is Jason still alive? Did he actually drown? Why did he spend all this time living in the woods? How could his mother who spent decades stalking the camp never notice he was nearby? Half the crew asked these exact questions, and the other half said not to think too hard about it.
Friday the 13th: The Game would later try to fix this via the Pamela Tapes. It still doesn't make a lot of sense, and its canonity is unknown. Jason didn't drown, but Pamela thought he did after several failed police searches and had a mental breakdown.
During the development it was planned that the sequels following the original one would have their own stories, and wouldn't have continuity with each other.
In the originally planned ending, Mrs. Voorhees' head was supposed to open her eyes before the credits start to roll. Sharp-eyed viewers can even tell that it's an actress in makeup rather than a prop. The sequence was scrapped because the producers felt the effect would be too cheesy.
Warrington Gillette auditioned for Paul but didn't get it. When filmmakers saw that he had stunt training, he was given some of Jason's stunt work. Stuart Charno also auditioned for Paul and later Mark, before getting cast as Ted.