Continuity mistake: We see a close up of Gabriella slipping in some milk, but in the wide shots there is no milk or carton on the floor. (00:41:30)
Deliberate mistake: At the start of the film when the commentators are explaining the tie situation between the 3 cars the green arrow to the car is out of alignment and misses the car, instead pointing at the track. Pixar Creative Director, Jay Ward, who is this film's team manager (as well as art coordinator) has stated that this was deliberately done as a joke, for fun. It's a little bit off, in the same way that Chick Hicks is a little bit off. This is the video with Jay Ward explaining: https://youtu.be/ZkJre-eC-zE?t=25.
Continuity mistake: Right before battle, Nasuada is showing Eragon Saphira's dragon armor. The helmet is full face, with two little slits for eyes. When it shows Saphira wearing it, the helmet covers only the top part of her head and cuts off above her eyes. (01:20:35 - 01:22:30)
Continuity mistake: In the scene when Jack Black is about to be unmasked and the kids are watching his match on TV, Chancho goes to to cover the TV. He is wearing a pink shirt but in the shot of him covering the TV he is wearing a white singlet.
Continuity mistake: When R.J. pulls the wagon to stop Verne, The ground hook and chain is next to the right front wheel. But while R.J. and Verne are talking there's no chain or hook to be seen until they start pulling the wagon from each other. (00:40:25 - 00:40:55)