Revealing mistake: When the chair moves across the kitchen floor, Craig T. Nelson goes to investigate it (after it has stopped moving). He goes up to the chair and flips it over. If you look closely, Craig almost hits the camera with the chair's legs. You can tell this has happened because the camera moves up very quickly and Craig looks straight at the camera crew with very big guilty eyes. This is more easily visible on the widescreen version of the film.
Continuity mistake: Likewise, when Carol Anne is being sucked into the closet vortex she clearly lets go of the white wicker headboard of her bed. In the next shot, the child (actually a dummy or a stunt double) is seen sailing into the closet, grasping a sizeable section of the now broken headboard.
Visible crew/equipment: In the breakfast scene when Robbie's milk glass spontaneously shatters from the bottom. Watch closely, you can see a string coming out of his cereal bowl and attaching to the bottom of his glass. Robbie just needs to give the glass a little tug, and splash. Watch in slo-mo a couple times, then it's really obvious at regular speed.
Trivia: The scene where the ghosts stacked the chairs on the kitchen table was filmed in one take with no cuts. As the camera followed Diane Freeling to the kitchen sink, the crew members rushed to the table, put an already built pyramid of chairs on the table and then took away the individual chairs.
Trivia: After the first Poltergeist movie was made, actress Dominique Dunne (Dana Freeling) was murdered by her estranged boyfriend in 1982. Actor Julian Beck (Kane) died of stomach cancer in 1985 after filming the second film. And after the third, actress Heather O'Rourke (Carol-Anne) died in 1988 from cardiopulmonary arrest brought by intestinal stenosis. This has (annoyingly) been called "The Poltergeist Curse."