This reigns as probably the most well-known movie curse, and it's practically a textbook definition of one. The basic story goes like this: A lot of actors who worked on the film eventually died unexpectedly. Once you take a close look at the events surrounding the movie, it also becomes a textbook example of why the idea of curses is silly.
The three original "Poltergeist" movies were released in 1982, 1986 and 1988, respectively. The two deaths that seem to give the curse the most traction are those of stars Dominique Dunne and Heather O'Rourke. Both died unexpectedly under surprising circumstances and at very young ages. Dunne, 22, was strangled to death by her ex-boyfriend in late 1982, a few months after "Poltergeist" was released. O'Rourke died in 1988, after filming for "Poltergeist 2" had been mostly completed. She had been diagnosed with the flu, but she actually had an intestinal blockage that led to a heart attack and ultimately sepsis. She was 12.
The deaths of Will Sampson and Julian Beck are frequently tied to the curse as well, but both died of long-standing medical problems. Sampson was 53 and died of complications following a heart/lung transplant in 1987; Beck, 60, died of stomach cancer in 1985.
All of these deaths are surely tragic and no doubt terrible for friends and family; however, four deaths related to a trilogy of movies doesn't even seem like a statistical anomaly. It's really the high-profile nature of Dunne's and O'Rourke's deaths that created this curse. Although actress JoBeth Williams claiming that real skeletons were used in the pool scene probably helped, too. In fact, plenty of people who worked on these movies are alive and well and enjoyed varying degrees of success afterward. Steven Spielberg seems to have done OK, and you'd think if there were a curse it would hit the producer/director hardest.