When Lorena is watching the videotape made of Steven apologizing to her for the message he left on her entrance door, you see her expression in several cutaways. The background showing her dining room to the right is always lit except for one cutaway in which the dining room is pitch black.
When Stephen brings home the dummy for the first time, he only opens the latch on the left side of the suitcase first. In the next shot, he opens both of the latches, even though the left would have already been unlatched.
When Steven is driving Fangora home she flicks her cigarette butt out the window as he pulls over to the curb. When they are in the car talking she is not smoking but when she gets out of the car she is smoking again.
The goof item below may give away important plot points.
Crew or equipment visible
In the last scene when Steven is dancing with Lorena he winks at his dummy and the dummy winks back, apparently autonomously. Lorena's daughter sees and gets up to check him out looking at his face and down his neck to see how he winked. Only you can just make out someone else's arm working the dummy in the bottom right hand corner of the screen, just as it winks.