Revealing mistake: As Drew Barrymore is running from the killer outside he stabs her. If you look closely it is very easy to tell it is a rubber knife, because you see it bend off of her. (00:10:20)
Revealing mistake: After Gale backs the van up, and Kenny's body falls on the windshield, they cut to a shot where Gale is driving away, and Kenny's body is on the roof of the van. When she starts driving you can clearly see Kenny lift his head into the air to look forward, even though he's dead.
Revealing mistake: When Sidney in Ghostface costume stabs Billy with the umbrella, you can see the squared shape of a protection pad under Skeet Ulrich's shirt (which she misses the second time around, famously prompting a genuine scream of pain from the actor). (01:39:55)
Revealing mistake: When the killer surprises Sid after she's come in from outside and locked the door, he tries to stab her with his left hand, misses, and hits the door right next to the chain in closeup. In wide shot (very quick shot) when Sid legs it, look at the killer and you can see he's holding a very badly bent knife in his right hand, nowhere near the chain. (00:27:55)
Revealing mistake: When Sidney and Stu are fighting in the living room and they both trip over the couch, Stu's stunt double is visible and wearing a wig. (01:40:25)
Revealing mistake: When Stu, Randy and the rest are talking about breasts, Stu looks down a girl's shirt. The girl smacks his shoulder and kind of grins at the camera, as if it wasn't supposed to happen. (01:12:55)
Revealing mistake: After Billy's stabbing, Stu charges Sidney projecting her through a doorway. In that shot you can easily see that Matthew Lillard is replaced by a stuntman. (01:40:15)
Revealing mistake: When Sidney is attacked by the killer for the first time (in her house), she is knocked to the floor and you can see she is played by a stunt double. (00:28:39)
Revealing mistake: When Sidney sticks her finger in the wound made when Sidney stabbed Billy with the umbrella, Skeet Ulriches skin pad visibly does not connect with his neck. (01:41:30)
Revealing mistake: When Sidney discovers Tatum's body, we see her head intact, in particular the eye level. In her death scene, her cranium was being crushed right at the eyesocket. (01:07:25 - 01:22:15)
Revealing mistake: Sidney is teasing Billy, and as a sort-of-reward to him she flashes him before he goes. That's the idea, but in the view from behind you can actually see through the nightie the shoulderstrap of a bra. (00:16:55)
Suggested correction: Could she have had a bra that undoes at the front and quickly flashed him?
To actually answer the question; her hand is on full display, holding the nightie open. She'd still have to open the bra. You can technically argue that you can't see her other hand, which she could be simultaneously maneuver to pull down the cup of the bra and expose a nipple, since there's hardly any time to undo it fully and she'd have to fasten it back afterwards. Or that cupless bras exist. Truth is, she opens her casual, normal, comfy nightwear during the unexpected visit of her boyfriend, and the way the scene is shot leaves no room to interpretation.
This is a question, not a correction.
