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.
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.