Audio problem: As Annie is trying to leave Joan's house and she starts to give Annie instructions on how to do the ritual, it's obvious the audio for Joan's lines was dubbed in in post as a lot of her speech doesn't quite match up with her lips. Especially when she's saying "Your whole family needs to be in the house". (01:08:00)
Continuity mistake: Meg starts the trip wearing a red flannel shirt over a blue cardigan over a gray T-shirt. Along the way she first drops the flannel shirt, then the cardigan (the color of which seemed to have gained saturation). She's not seen carrying them or any bag that might have held them. When she finally faces It, all the clothes are back on. Calvin's case is even more evident: he starts with a jacket over a red shirt, ends up with a just a now-gray T-shirt (matching Meg's) but is back with jacket and red shirt at the end.
Continuity mistake: As Keda is tumbling down the face of the cliff, he smacks against a small ledge on his way down before hitting a second ledge and then landing on a third. But as the camera goes down the face of the cliff from the top of it, the two ledges he hit on the way down are gone, showing a mostly flat face. (00:28:20)
Other mistake: When he arrives at the club he texts "I'm here" and receives the reply "bathroom" but the texts are on the wrong side of the screen. It looks as if he replied "bathroom". (01:20:10)
Continuity mistake: At the school, when Scott's suit is malfunctioning and he's big in the janitor's closet, his head is tilted towards his right shoulder. When Hope is working on the suit, we see Scott's head is now tilted towards his left shoulder, even though he's stuck and wouldn't have been able to move around like that. But in the next shot it's back to his right.
Plot hole: As shown when Robin comes under the influence of Slade's mind control device, the hypnotized people retain their abilities and higher intelligence, just with a loyalty to Slade. That being said, when Slade hypnotizes all the DC heroes, Superman and The Flash are among them in the theater. Slade orders all the heroes to chase after and destroy the Teen Titans. The Titans take off on foot with the others running after them and are able to keep their distance in a big chase sequence. Superman and The Flash never use their super speed to over take the Titans, which would have been so very easy.
Revealing mistake: During the police chase with Gabriel on his motorcycle, he stops and turns to start shooting at one of the cops. By appearances, Gabriel is using a standard Glock which had a magazine capacity of 15 rounds. He has no extensions on it at all with the handle and magazine visible. Yet he proceeds to fire the gun 33-35 times at the cop car without reloading and in continuous fire. (00:57:55)
Continuity mistake: As Lara is headed up to the boat's pilot house, she obviously leaves the hatch open, but a far away shot shows the door closed again, as she's climbing the ladder. When she goes back down, it's suddenly open again. You could claim it's the rocking of the boat, but as she's grabbing her gear, the door stays open.
Continuity mistake: After Gillick saves Isabel and they get into the car, the seats in the car don't have any head rests. But after they turn, and the camera shows them in the car again, the headrests are there. (01:02:55)
Other mistake: The snow globe from the original is in the loft in the new film, but in the original film, as Mary was leaving the Banks house while they flew a kite, she packed it in to her carpet bag.
Continuity mistake: When she is in the boat she takes off her blindfold entirely, but after some talking the blindfold is around her neck all of a sudden.
Character mistake: When Logan tries to use both his and his mom's phones after her fingers were broken by the Man in Black, he throws them in disgust when he finds the SIM cards have been removed. However, every cell is capable of placing a 911 call regardless of whether a SIM card is present or not.
Continuity mistake: At the beginning of the movie, the teacher asks "Has Nicky's mom volunteered?" The guy answers "Emily?" with his arms down his hips, but he had his arms folded across the chest just in the previous shot. (00:03:50)
Continuity mistake: When she is painting the wall and stops to go and hug him, in different shots during the hug the amount of white she has painted lessens, and has a different pattern. (00:07:45)
Continuity mistake: After Elizabeth drives her motorbike off the peer, across the frozen lake and stops while she looks back at the police, the Ducati suddenly has different tires than earlier in the escape sequence. (00:25:55 - 00:27:05)
Plot hole: Kingsley holds the only key card that can access the control room where the Captain is. He only has one card. He gives this key card to Rhodes so he can go up there and talk the Captain into not taking the sub to the surface yet, while Kingsley and his men head down to stop Scott and Taylor. When they employ the help of Marco and take out Kingsley and his men, Marco bends down and pulls the key card off Kingsley's neck so that now they have it... only Kingsley didn't have the key any more, as Rhodes had it and was still in the control room. A second key card magically materialized on Kingsley's neck.
Suggested correction: Kingsley only took the keycard from his neck to open the door and then hold it in his hand, so I guess he actually didn't give it to Rhodes. Just opened the doors.
Factual error: The helicopter used during the exchange is a Mi2, which has not been in service with the Russians since the 1970s.
Continuity mistake: At the end of the taxi chase where the final biker is hit by the bus, the bus is shown moving at speed, and easily clears the junction where the taxi halted before you heard tyre squeals to signify it stopping. Yet in the next scene as the crowd gather around the presumably dead biker, the scene is shown happening in the middle of the junction, with the bus stopped almost before it's even entered the junction.
Continuity mistake: The blue paint blob on the painting changes between shots. (00:47:00)