(at around 6 mins) When Ghostface is after Christine his knife makes a large hole in the door. When Cotton later approaches the door the damage is a series of narrow slits.
Shape of the cuts on Dewey's forehead throughout the film.
Roman Bridger's sideburns shorten and lengthen in the final scenes.
(at around 4 mins) Christine gets out of the shower and steps on the towel, but when she gets dressed and starts to walk down the hallway, her feet are still soaking wet.
(at around 1h 19 mins) When Gale reaches for the voice changer, she has blood on the top of her hand. When she puts the voice changer to her mouth, there is no blood.
In the beginning of the movie, Cotton is talking to the killer on a cell phone and when he gets hung up on, a dial tone can be heard. Cell phones don't make that dial tone sound either before or after a call.
At the Stab 3 wrap-up party, Ghostface cuts the power to the house and uses the fax machine to send script pages to everyone inside. If the power was cut, then the fax machine would not be working.
Right before Gale and Dewey find the voice changer, Gale tells Dewey to use the caller ID on his cell phone and dial the last number in it, he does, causing the "killers" cell phone to ring, Dewey hangs up his phone as soon as he hears this, however the other phone never stops ringing.
(at around 1 min) When Cotton is on two wireless telephones at the same time, he puts one phone on hold even though the brand and model does not have a hold feature.
(at around 44 mins) In the "....whoever smells the gas" scene the killer has cut the power to the house, but the pool lights remain on and the fax machine works. An uninterruptible power supply (U.P.S.) is plainly visible next to the fax machine, which may be unusual, but certainly makes it possible for faxes to be received and printed.
(at around 9 mins) In Sidney's first scene, she enters 1288* on the alarm-system keypad on the road gate, and then twenty seconds later enters 1288# on the alarm-system keypad inside her house on the wall next to the door. It is entirely possible that 1288* arms the road-gate alarm and 1288# arms the house alarm.
(at around 1h 5 mins) When the killer chases Sidney through the set, Sidney opens a door that leads nowhere and climbs through it to the side of the set out of view of the camera. The killer would have known the door goes nowhere, but it would not have stopped the killer from opening it. The killer falls because Sidney pulled the killer's shirt.
While talking on the phone, Sarah mentions the shower scene from "Vertigo". The shower scene from a Hitchcock movie was from "Psycho". This is obviously a jab at her "dumb blonde" persona as the series is well known for its numerous pop culture references and is unlikely to so obviously mix up two of the most famous and respect films.
(at around 6 mins) When Christine is running from the killer, she locks herself in a room. The killer makes a single thin linear hole in the door with his knife, but he then twists the knife, turning it into the circle we see later.
(at around 1h 28 mins) When Sidney inspects the newspaper clippings in her police file, it's clear upon closer inspection that the stories have nothing to do with their headlines.
(at around 1h 23 mins) When Ghostface throws Tyson off of the balcony, his leg flashes in the bottom-right corner as he lands on a mat.
(at around 57 mins) When Jennifer uses her card to gain access to the Sunrise Studio Archives she clearly swipes her card the wrong way.
(at around 39 mins) When Gale is eavesdropping on Dewey and Jennifer talking about her, when Dewey says ".. I left out the part where she's cruel and selfish" his lips clearly aren't moving.
(at around 1h 30 mins) When Sidney is in the police station, she's on the phone with the killer on a cell phone. When the killer hangs up, you can hear a dial tone. You wouldn't be able to hear a dial tone on a cell phone.
(at around 1h 26 mins) You can faintly see the wire going across the screen holding the spinning knife right before it hits Dewey in the forehead.
When Sidney, Dewey and Gale meet Martha, Gale does not know who she is. However, earlier in the film, Dewey and Gale discuss how Gale lived with him in Woodsboro until after he recovered from the injuries he sustained at Windsor College. Therefore, Gale would no doubt have met Martha, a close friend of Dewey's, already. Martha even tells them: "We miss you in Woodsboro, guys."
How did Ghostface manage to track and call Sidney's number while she was hiding in isolation? According to the film proper, Sidney went into hiding to deal with her traumas and not even Detective Kincaid was able to find her. A throwaway line suggests that Ghostface used scanners to find Sidney's number and monitor her calls, but that still doesn't explain why he was threateningly asking Cotton where he could find Sidney.
Dewey's gun mysteriously vanishes after Ghostface punches him in the face during the attack with Gale, Jennifer, and Tyson, but he suddenly has it again during Jennifer's death scene as he's shooting the glass.
(at around 23 mins) When Sarah Darling is in the empty production offices of Sunrise Studios, she is looking for Roman Bridger, the director of Stab 3. However, when Sarah calls for Roman in an office, she clearly says, "Ronan," not "Roman."
Roman using his normal voice to talk to Sarah before switching to the Ghostface voice was a stupid move that could have blown his cover. Yes, the killer does use a voice-changing device to use other people's voices to trick their victims but Sarah doesn't know that, only the audience does. If Sarah had survived this encounter she could have easily blamed Roman for the attack, potentially jeopardizing his plans.
The Security Guard locks Sarah Darling inside Roman's office assuming it's empty, trapping her inside. How did he not see Sarah's car parked right outside the door? That should have been a giveaway that there's still occupants inside the office.
Jamie Kennedy accidentally says "chrapter" when saying "concluding chapter of a trilogy" seemingly excited to say trilogy and instead adding 'tr' it to chapter.