(at around 1h 36mins) When Dewey informs Jill about Sidney's situation he says "Jill, they're not sure yet. It's still touch and go. But she's in ICU, and they think Sidney just might make it", but in the next scene, when Jill goes to Sidney's room to try to kill her, Sidney is awake and ready to put on a fight.
(at around 56 mins) During the "Stabathon", when "Stab" is playing, the scenes are out of sequence. The first scene shown is the popcorn burning, then the shower scene. The shower scene is supposed to be in the opening sequence, before the popcorn burning scene.
(at around 1h 30 mins) When Jill turns to throw herself on the glass table, her hair is over her shoulders. The next shot where she actually throws herself onto the table, her hair is over and down her back.
(at around 10 mins) When the girl crawls away from Ghostface while bleeding, he moves to the garage door controls to close the door while she's crawling under it. We see a shot of the girl crawling, and in the last second before the camera shifts, the garage door starts to go down seconds before the camera switches to Ghostface hitting the "Close" button on the controls.
(at around 1h 28 mins) When Charlie shows Sidney the footage of Ghostface stabbing the girl in her bed on the cellphone, it completely contradicts the earlier shot of Jill removing the camera from the first person perspective from inside the mask.
(at around 36 mins) When Sidney runs to Olivia's house she walks into the blood splattered room. Sidney runs her hand down the blood covered door frame. None of the blood smudges or goes onto her hand. If the murder had just happened the blood would have went onto her hand, and smeared down the door frame.
(at around 1h 6 mins) When Sidney is unpacking her aunt's groceries, the box of cereal has already been opened and closed back shut.
(at around 54 mins) When Kirby is on the phone to Jill, as she approaches 'Stab fest' party, her iPhone is clearly on the contacts list and not on call mode. iPhone screens are blank during phone calls when up against the ear.
At about 1:15, when Robbie runs into the hanging plant, his camera is on the right side facing forward with the horizontal pivot/mount on bottom, when he puts it back on it is still on the right with the pivot on bottom, but facing backwards. If the camera was rotated horizontally the pivot would be on top, and if put on backwards it would be on the left. The camera is still facing perfectly forward after being knocked off and perfectly backwards when picked up, an impossible vertical rotation from the landing.
Sheriff Riley and his deputies wear six point stars as a badge, yet on the sleeves they have seven point star patches. In any real Sheriff's department one or the other might be used, but never both. The uniform insignia would match the badge.
(at around 16 mins) In the beginning of the movie, when Gale is sitting in front of her 6th generation Apple iMac, instead of OS X 10.6 Snow Leopard, it's got OS X 10.4 Tiger installed. Every 6th gen iMac came preinstalled with Snow Leopard.
(at around 17 mins) When Gale is typing at her computer, she continues to press keys even though the screen shows that she had finished the sentence she was writing.
Neither Charlie or Jill would be physically capable or throwing Marnie's body through the window at the start if the film. Similarly, when Rebecca's body is thrown off the building it lands on a van parked several feet from the side of the building.