When Eric pulls Tina from the trash can after he agrees to return her to Christie, he unwraps the cord from around her neck twice.
When Erich finds the doll sitting in the garage and picks it up, the doll's legs are bent as she was sitting. Next, we see her legs straight as he's holding her but when he throws her into the trash, her legs are bent again.
When Eric is trying to burn the doll with the blowtorch and the flame keeps going out, in the wide shots, you can see his fingers release from the trigger every time the flame goes out.
When Erich picks Tina up off of the table saw, briefly visibly is a metal band around her neck, presumably to create the sparks from the saw.
When Erich answers the phone the shadow of a crew member or equipment can be seen on him as the camera moves in.