Complete Monster: Reverend Henry Kane was the insane leader of a utopian cult in the early 19th century who led his "flock" into an underground cavern under the premise that the world was about to end, but in actuality, his only intention was to harvest their souls for his own power. As a ghost, he absorbed the energy from his followers, which fused with the evil in his heart and turned him into a monstrous apparition—a demon called "The Beast". When the Freeling family moves into the home built where the cavern once was, Kane tries to abduct the young Carol Anne and bring her to the realm of the dead, dubbed the "Other Side," where he could use her to attract more souls for his energy. In Poltergeist II: The Other Side, he keeps on tormenting the Freelings, possessing Steven's body in order to rape Diane while taking a proactive role in psychologically and physically tormenting the family as he seeks to take Carol Anne. At the end of Poltergeist III, Kane apparently abandons his pursuit of the Freelings and accepts the mystic Tangina's offer to show him into the light, but the film ends with a flash of lightning and Kane's demonic laughter, showing his supposed redemption as a sham.
Director Displacement: It's directed by Tobe Hooper, not Steven Spielberg. Interestingly, this popular belief might be nearer the truth than expected: Spielberg produced, wrote and storyboarded the film, to the extent some actors of the film have went to say he directed more than Hooper himself (Zelda Rubinstein claims that Spielberg directly directed her scenes, although other lead actors gave accounts that differed on who did what). All sources are in agreement that Hooper had little to no involvement in post-production, with the sound editor claiming he received no input from him and Jerry Goldsmith saying he only interacted with Spielberg, never with Hooper.
The last twenty minutes of the film feature Jobeth Williams being thrown around in her panties and a football jersey, which keeps getting pulled up; then running outside and getting muddy when she falls into the pool; then a bunch of coffins rise from the ground, and their corpses, actual corpses mind you, fall out turning the thing into a bowl of dead guy soup.
The scene with the football jersey becomes this when you realize that it is not just a cheap joke, but that the Poltergeist is trying to rape her.
An odd meta-example. The scene near the end where an invisible force pulls up the mother's night-shirt and exposes her panties gets laughs from kids. However when seen as an adult, you realize that the poltergeist is attempting to rape her. (In the screenplay it actually does).
The clown strangling Robbie when you hear that Oliver Robbins was really choking during the filming of that. Had the filmmakers not intervened in time, that would have made it all three of the Freeling children's actors who died young.
An early draft of the script had Carol Anne dying in the first act and haunting the house, but they changed it for being too dark. Especially uncomfortable that Heather O'Rourke passed away at the age of twelve.
Steve's angry outburst towards his boss at the end when the corpses of the cemetery their house was built in start bursting from the ground during the final meltdown, revealing that he'd only moved the headstones and left the bodies exactly where they were is rendered unintentionally funny by Craig T. Nelson's hysterical delivery:
"You only moved the headstones! WHYYYY?!! WHYYYYY?!!"
The constant screaming at the end can veer off into this for some viewers.
When Steve/Nelson has packed his family into their car to flee the self-destructing house, he frantically paws at his rear pockets looking for his car keys while the camera focuses on his butt, making it look for about two seconds that Steve has decided to do a brief booty dance.
From the 2015 remake: when Amy first hears Maddie from The Other Side, and says to her "I miss you so so so so so so so much"... It's the over-repetition of about a dozen "so"'s combined with her performance that makes it funny instead of dramatic.
Narm Charm: Tangina's voice is funny at first, but then you get used to it.
Values Dissonance: The construction workers who are digging the pool hit on Dana as she heads out for school (and, mind you, she's wearing her uniform, so she's very obviously a child). It's Played for Laughs as she elaborately flips them off and seems no more than annoyed, but these are still three adult men catcalling a sixteen-year-old.
It's the Same, Now It Sucks!: An incredibly common criticism of the remake; it seems to have been made with little purpose but to cash in on the Poltergeist name, so it just rehashes the original and calls it a day.