Visible crew/equipment: At the end when Sidney's dad falls out of the closet, if you look closely you can see the hands of a crewmember who was in there too, pushing items out after him. (01:42:40)
Visible crew/equipment: When Casey tells the killer "Dial somebody else, okay?", you can see the shadow of some equipment on the decorative pillar at the top right of the screen as the camera moves backward. (00:03:18)
Visible crew/equipment: When Casey agrees to play the killer's game she crouches down next to the television and as the killer says "Here's how we play..." you can see a clear shadow of the head of the boom microphone cast on the white wooden railing at the top right of the screen. (00:06:25)
Visible crew/equipment: Look closely (you may need widescreen to see it) at the railing when Sidney is walking up the stairs outside in her parent's wooden patio talking to Tatum on the phone. You'll notice a quick boom shadow on the right of the camera as it's backing up. [Noticed also by Wes Craven on the DVD commentary track]. (00:22:25)
Visible crew/equipment: There is camera equipment reflected in the door window when leaving the police station. (00:36:10)
Visible crew/equipment: When the officers take Billy away from Sidney's house, a set of lights is reflected in the back window of the car. (00:31:00)
Visible crew/equipment: When Drew Barrymore runs away after the killer just called her 'blondie', there's nobody behind her character, and yet a new shadow is cast on the wall to the left when the camera follows her around the corner to lock the front door. (00:03:50)
Answer: This is according to IMDb: When Sidney comes out of the closet and stabs Billy with an umbrella, the stunt man was supposed to hit a pad on Skeet Ulrich's chest. The first hit got the pad but the second one slipped and hit him in the chest (you can see it in his reaction). Wes Craven kept it in because of its authenticity.
Shannon Jackson