Here's why unlucky teenager Scott is left trapped on the "other side" at the end of Poltergeist III. The third entry in the horror series saw lead character Carol Anne (Heather O'Rourke) being sent to Chicago by her parents to live with her aunt Pat (Nancy Allen). The third entry makes impressive use of the John Hancock Center - where the entire film is essentially set - and features some genuinely impressive practical effects. The movie is also considered the worst installment of the original Poltergeist series, and suffers from a weak story and lack of scares.
The movie was overshadowed by the tragic passing of star Heather O'Rourke, who died at the age of 12 due to an illness shortly before the movie's release. Poltergeist III director Gary Sherman originally didn't want to finish the movie at all, but when MGM made it clear they would complete it with or without his input, he returned. A new ending was quickly shot to wrap up the story quickly, with Carol Anne - whose face is never shown - being played by a double.
One thing Poltergeist III's ending doesn't resolve is the fate of Scott, the boyfriend of Lara Flynn Boyle's Donna. Earlier in the movie, Scott (Kip Wentz) is taken to the "other side" by villain Kane's evil spirits, but he later emerges frozen from a swimming pool. This Scott is later revealed to be an evil doppelganger, and in the finale of the movie, medium Tangina (Zelda Rubinstein) brings Kane into the light, freeing the captured Carol Anne, Donna and the rest of the family from the other side. Noticeably absent from the ending, however, is Scott, which has a couple of explanations.
Sherman had previously shot another finale for Poltergeist III he was unhappy with, and which tested poorly during screenings. This ending featured the frozen bodies of all the captured characters, including Scott, but Tangina's sacrifice frees them to return from the other side. This ending also featured Heather O'Rourke and Scott actor Kip Wentz, but before O'Rourke's passing, it was planned for the crew to return and film another ending.
A devastated Sherman reunited the cast and crew to quickly film the somewhat abrupt finale that now ends Poltergeist III, which he's also unhappy with. According to the director, Kip Wentz was unavailable to shoot because he was studying outside L.A., where the reshoots happened; Wentz has since disputed this, stating he was in the city and available but he was just never contacted about returning. Sherman reiterated his belief Wentz was unavailable during a 2021 interview with podcast Post Mortem with Mick Garris, but also recalls when he was told the actor wasn't free that when offered the choice to reschedule and wait for him, Sherman said "F*** it lets just get this over with, nobody will miss him."
A miscommunication may be what led the Poltergeist III production to believe Wentz was too busy to make the dates. Many of the crew were still reeling for Heather O'Rourkes passing too and wished to just get the reshoot over with, so it's also likely that resolving Scott's fate wasn't considered a priority, which Sherman's podcast interview seems to make clear. Hopefully, for Scott's sake, he was spat out of the other side in a different part of the building instead.