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 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 1h 5 mins) When Sidney makes a cup of tea before taking the wind chimes down at her aunt's house, she places the yellow tea kettle on the cutting board (blocking the lemon and knife blade from view). After she takes down the chimes and returns to her cup of tea, the kettle is not visible but the lemon and knife are. Shortly thereafter you see the kettle is placed on the middle of the kitchen island.
(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 9 mins) During the opening, when Jenny is talking with the killer (just before Marnie is thrown through the back-porch door) she is clearly standing in the kitchen and by the stairs, but when Marnie's body flies through, Jenny is back in the living room by the door.
(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 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.
The goof items below may give away important plot points.
(at around 46 mins) When Rebecca is running towards the exit door in the parking garage, there is an exit sign on the door. When she gets stabbed, the sign had disappeared, and the cars are parked in an other way.
(at around 1h 29 mins) Jill is stood holding the gun and the knife and is waving the knife in Sidney's face. But, when her and Charlie go to stab each other she's only holding the gun and he passes her the knife.
(at around 1h 28 mins) When Charlie holds his iPhone up to Sidney to show his murder footage, he's holding it with his hand vertically with his thumb on the bottom and four fingers on top. In the next shot when Sidney watches it, he's holding it up to Sidney with his hand horizontally: using only his thumb, index, and middle finger.
(at around 1h 35 mins) When Jill comes into Sidney's hospital room she pulls out the call bell cord before Sidney can call for help. In any hospital if you pull out the cord it sounds an alarm to call for help as well, yet no alarm was heard.
(at around 1h 11 mins) When at Kirby's house after Sheriff Riley interrupts the Stabathon, Charlie says he will watch Stab 7, and puts on the DVD. But when we see the film playing later, it is the girls from the opening scene, the ones with the Facebook-stalker, which was revealed to be Stab 6. Yet as discussed by Jenny and Marnie in the 'true' opening, the beginning of Stab 7 had Rachel and Chloe watching Stab 6 and criticizing it.
(at around 1h 28 mins) When Charlie and Jill reveal themselves, Charlie points to Jill with his knife, indicating her to be the "new" Sidney. When he points to himself, you can see the ridge where the fake blade retracts, showing it to be a prop safety knife.