Revealing mistake: During the dance number in the airport, Santa grabs a security guard who then starts break dancing. It's obvious this dancer is a stunt double as he's younger and skinnier than the real guard.
Revealing mistake: When Devil Face cuts J.D's arm the knife's clean. (00:52:47 - 00:53:10)
Revealing mistake: During the first chase, when the Batmobile joke is said, skidmarks from previous takes are visible on the road. (00:02:55)
Revealing mistake: When the shop teacher walks up to Millie at the start of the film and slams the book, as he begins to step in front of her table, you can see him subtly "step down" even though he's on an even floor. Due to technical limitations, there needed to be a raised surface on the floor for the camera, a few inches off the ground. It was painted out digitally, but you can still see the actor step off the platform, making it look like he's "stepping down" on even ground. (Confirmed in Blu-Ray commentary.)
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)
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: In one of the scenes in the ballroom you can see that the thumb claw of the head witch is put on top of her real thumb.
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: 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: 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: When the black cat was going up the staircase and got to the first landing, it turned its head to look directly at the camera, then continued climbing the staircase. (00:08:13)
Revealing mistake: It looked like Theo's gold and white checked shoes were a few sizes too big and wide to make it easier and quicker for Laura to put them on her, but Laura still had some difficulty slipping on Theo's oversized shoes. The material even folded down around the ankle when Laura was putting on Theo's right shoe. (00:03:42)
Revealing mistake: When De Niro/Grandpa falls down the roof and hangs on to the drain, he is replaced by an obvious stuntman with a wig and thinner build.
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: 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: Men of the Department of Sanitation are picking up Lita's body from her house. The body bag she is supposedly in moves so easily, you can tell it is very light and contains nothing very heavy, much less the weight of a body. (01:03:05)