Factual error: While bidding on eBay for the steering wheel, Vanelope tries to up the $350 bid by bidding $1,000. This scares off the real bidder, but Ralph and Vanelope don't understand how eBay works and bid against each other until they win with a $27,001 bid. That's not how eBay works. Vanelope's initial $1,000 bid wouldn't have set the high mark, but would simply have been her maximum bid in case the other bidder kept increasing his bid. The current high bid would have increased by the increment amount until it reached Vanelope's max bid. Now, it could be that the system recognized Ralph and Vanelope as separate bidders and that's why it let them keep upping the max bid. But, if the system had responded normally and simply increased Vanelope's bid to $355 with a max of $1,000, they might have realised what was going on and not started bidding randomly.
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)Quantom X
Other mistake: As established in the original and this film, when the Admiral fires his cannon, everything in the house falls off walls and mantles, and everything needs to be caught to prevent them from being damaged. However, when he fires his cannon as Mary Poppins and the children are going out to have the bowl repaired, Michael has already left for work, Jane and Ellen are standing outside, and Mary, the children, are getting onto Jack's bike - leaving no-one in the house to catch the falling objects which would therefore all be smashed or damaged.