Revealing mistake: In the scene where Dave (Alan Tudyk) is brushing off Sid (Tony Hale) at the reunion, the camera from slightly behind captures jaw movements that don't match his speaking, and he continues speaking when there is no jaw movement at all. (01:47:00)
Revealing mistake: The backgrounds during the pawn shop scenes often change, suggesting the use of a "green screen." For example, the grandfather clock was behind the clerk to his left, then not there, but back again. Sometimes shirts are hanging in the front windows but other times there are other items there. (00:47:57)
Revealing mistake: When Cecilia is lifting an iPhone up to her ear, it starts ringing as if she's making a call. But on the screen, you can very easily tell it doesn't say "dialing" or "calling" - it's a timer showing how long the call has been going on. She was already on a call before pretend-calling. (01:42:47)
Revealing mistake: The location of and distance between windows on the mobile home should stay the same, but do not. The home's exterior is constructed of vertical boards/slats, which can be used to make a rough estimate of the number (then distance). Initially, there were about 24 vertical boards between the shortest window and the window to its right (on-screen). When David and Anne were close to the house, there were a lot more boards between these windows - closer to 36+. The mobile home was "fabricated". (00:02:18)
Revealing mistake: The green screened scene behind the ticket collector on the dock isn't moving. The waves on the water are paused, then the background is moving again in the next scene. (00:08:00)
Revealing mistake: When Maya Rudolph and Tim Meadows are in their car at the drive-in theatre, whilst Maya Rudolph says "We got Hubie Dubois spying on everybody", there is a close-up shot of the wing mirror with Hubie's car reflected in it, along with several other cars and a building in the background also reflected. Despite being a reflection, any text or numbers reflected in the mirror are not reversed. The number plate on Hubie's car is spelled out forwards, and the sign "Salem Drive In" on the building in the background is spelled forwards also, and not reversed as they normally would be in a reflection. (00:49:17)
Revealing mistake: The Uber car's instrument panel did not appear to be functional (indicating it was fake). On the driver's side, the numbers in the two large circles were not discernable, but one had to be the speedometer. On the left side, the arrow pointed to maybe 6 MPH, even when the posted speed limit was 60 MPH. The gauge on the right side stayed at the bottom of the left side at zero. (00:09:50 - 00:13:05)
Revealing mistake: Clearly not Vince Vaughn during the opening kills as the Butcher seems leaner than Vaughn.
Suggested correction: Seems leaner" is just far too vague to be a mistake, and I didn't see anything that made me immediately think he was "clearly not Vince Vaughn.
Revealing mistake: In the opening pre-calculus class, Mara bending over to the side of her desk is what enabled her to avoid being splattered when Kaitlyn - sitting in front of her - "exploded." It was also a cue that the "blobs of blood" were about to be strewn across the classroom. The boy seated at the last desk behind Mara moved his right hand up to his forehead (similar to how one would block sunlight) in anticipation of getting splashed with blood. After some of the "blood" landed on him, he quickly moved his hand away from his face. (00:00:35)
Revealing mistake: When the (imposter) female security guard was lying on the floor with her head slightly raised and shot herself, the gun was about parallel to the floor; the blood splatter should not end up primarily mid-way up the wall like it did. The floor-level view of the dead guard shows a reduced-size wall so that the blood splatter looks much closer to the floor than it was. Also, at least some blood should be visible on the guard's temple but none was. (00:35:40)
Revealing mistake: As impressive as the CGI lions were, there are almost always giveaways. For example, when Fareeda calls Charlie over and goes to hug him, her left hand and forearm quickly disappear into his mane and go too far in to be realistic. There is also a small but noticeable gap when Charlie's right paw is curled over Fareeda's left shoulder. (00:18:45)
Revealing mistake: Throughout the movie, there are mixtures of real snowfall and artificial flakes. When John is at Paul Carnury's house, in particular, the "fake" snow is quite apparent. John could be standing outside in the snowstorm yet little or no snow hit his jacket. (At one point, a few flakes land on top of his head.) When John breaks through the front door, a gush of "flakes" come inside and the ones on the back of his jacket fly and/or roll over his body. "Snow" even bounces upwards off his jacket. (01:13:34)
Revealing mistake: Vic jumped onto the roof of the white van while it was moving. The van driver started weaving to get Vic off the roof - but Vic could not be seen at all when there was a front view of the van. Even if Vic were lying on the roof, at least part of him should have been visible. (01:04:04)
Revealing mistake: Near the beginning of the movie, there does not appear to be a curtain (or window) on the left side of the screen (e.g, when Samantha is sitting on the couch), but there is near the end when Chloe enters the now-empty room. The use of a green screen seemed apparent throughout the movie. (00:04:19 - 01:23:04)
Revealing mistake: The nefarious villain has just revealed to Prince Charming that he has kidnapped the princess, and shows her to the crowd as a big burly smurfy monster is holding her. The crowd in unison gasps and looks horrified...except an extra who just can't keep a straight face. (00:07:35)