Continuity mistake: At the end, when Sam looks at the mask, then drops it, when we see it on the ground, there's suddenly a bit more blood on the mask than the previous shot where we saw it.
Continuity mistake: When Sam and Tara sit down on the stairs after killing Bailey, Sam rests her head against her hand. After this happens, her mouth goes from open to closed between cuts.
Continuity mistake: From when Detective Bailey wakes up to when he is killed by Sam, the blood pattern on his face is mostly consistent, but the volume of blood changes a few times. It was likely taken from multiple takes, and whenever they redid his bloody makeup, it wasn't quite an exact match.
Continuity mistake: As Sam and Tara walk away at the end, in the first shot, they are shoulder-to-shoulder. When it cuts to the wide shot starting at the mask, you can see they are instantly standing about 3' apart.
Continuity mistake: When Chad is stabbed, the little streams of blood running from his mouth and down his chin change positions a few times between cuts, revealing multiple different takes were used.
Continuity mistake: When Ghostface is about to start taunting Gale about how Dewey was killed, at one point, a piece of her hair falls down over the phone she's holding to her ear. In the next shot, it vanishes.
Continuity mistake: When Sam and Mindy barricade the bathroom door that Ghostface is trying to kick in, the amount of blood around the sliding bolt lock changes repeatedly between cuts.
Other mistake: When Anika falls to her death, as she lands, there's already a moderately-sized puddle of blood beneath her, which makes no sense, as she only just landed that instant. Then, when it cuts immediately to a wider shot about two seconds later, there is suddenly even more blood around her body.
Continuity mistake: When the "core four" are at the table and hear Quinn in the other room, Tara says, "Quinn and her gentleman caller are back at it again." As she says this, she goes from holding a spoon in her one hand to holding nothing and having her hands clasped together between cuts.
Continuity mistake: When the "core four" do their first high-five/hand-joining at the dinner table, the way their hands are intertwined changes when it cuts from the wide shot to the close-up. Most noticeably, Tara's fingers go from being interlocked with Chad's fingers to gripping the side of his hand between cuts.
Continuity mistake: When Sam walks into the kitchen with Tara, Chad, and Mindy while they're cooking dinner, if you pay attention, the wooden spoon is in the pot still in the wide shot, but when it cuts to the medium shot, Chad has it lifted about 1' in the air above the pot.
Continuity mistake: When Tara tells Chad that she thinks he should get out of the city while they're cooking in the kitchen, Tara's hair moves on and off her left shoulder several times, depending on the camera angle.
Continuity mistake: When Bailey and Kirby are looking at the corkboard with all the previous killers, watch closely when Kirby's phone rings. Between cuts, she goes from looking forward to looking down. Then, in the next shot, she goes from looking slightly to her right to looking slightly to her left. It's subtle and can be easy to miss but noticeable once you see it the first time.
Continuity mistake: When Bailey shows Kirby the photo of Sam's dead therapist and says, "He was stabbed through the nose," pay attention to Kirby's mouth when she grimaces. She goes from having her mouth closed to open between cuts.
Continuity mistake: When Mindy is giving her speech about franchises, when she says "Oh, and that's not even the worst part," her hands go from together and up near her abdomen to separate and down by her sides instantly between cuts.
Continuity mistake: When Mindy is giving her speech about franchises, when she talks about how "nobody just makes sequels anymore," when it cuts from the wide shot to the medium shot, the way Chad is holding his pencil and the way Ethan is holding his knee changes between cuts.
Other mistake: When Sam and Tara first meet Gale outside, after Gale gets punched, pay attention to the multiple news crews behind her as they start arguing about Gale's book. Randomly, they go from paying attention and recording to suddenly all dispersing and packing up after the shot cuts to Sam and then back to Gale. This makes absolutely no sense. They're all involved in a very famous case; there's no reason every reporter there would suddenly start packing up in the five seconds they were offscreen.
Continuity mistake: When Sam and Tara are at the police station, pay attention to the objects on the table. After officer Bailey enters the rooms, several of the objects on the table suddenly change positions from the first shot. For example, Tara's phone and coffee cup swap places. The pen has moved a few inches up on the notebook. A few of the photos shift around, etc.
Continuity mistake: When Sam and Tara barrel into the corner store, you can see a bus on the street behind them. However, no bus was visible in the previous shot anywhere on the street as they ran up to the store. True, you don't see the entire street, but you absolutely should have been able to see the bus somewhere in the previous shot.
Continuity mistake: When Ghostface grabs Dr. Stone through the glass door and starts smashing his head against it, the way the glass is broken changes in almost every single shot, revealing multiple different takes were used.
Answer: I think this is more "trivia" than mistake. There's no error in a film taking place a bit further ahead in time than in its release date.
TonyPH