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. (00:33:30)
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.
Revealing mistake: When the father tucks Robbie in and Robbie says "I don't like the tree, dad." the camera shot begins rolling to the left. Robbie flinches when reacting to someone telling him to lean back more so he's not blocking too much of the father's face. The father also improvs a solution by gently pushing Robbie's shoulder back a bit as well. (00:18:55)
Continuity mistake: When they put Diane and Carol Anne into the bath tub, near the end of the film, look at Carol Anne's face. The red gunk changes in pattern every time we look at it.