Continuity mistake: Before Lightning falls off the end of Mack's trailer, you can see Snot Rod and one of the other cars running alongside the trailer. But when the camera moves to show Lightning coming off the trailer, you can see alongside the trailer in the background, and the cars are now missing. Camera switches back to the front of Mack, and the cars are back alongside again.
Continuity mistake: In the tractor tipping scene, Mater demonstrates to McQueen how to do it using two tractors before Mcqueen's first try. And then when McQueen accelerates to make the noise and we have a wide open shot of the field, we can see that all the tractors in the field are still turning. Missing are the two already done by Mater.
Other mistake: After the first race, in the scene where Lightning is rushing out on the stage thinking he is the winner, loads of confetti is shot out when the King and Chick Hicks gets onto the stage. The confetti is then falling down, but not a single flake hits any of the cars. However, in the following shots, you can see plenty of it on the stage underneath the cars, even though they haven't moved.
Deliberate mistake: During the very last race, Lightning McQueen gets next to the green car, and the reflection of that green car's chrome "C" emblem is seen on Lightning McQueen's hood. However, the C is facing the right way. Since the C is placed on the green car in the correct direction, the reflection on Lighting McQueen should be a mirror image.
Other mistake: When Mack wants to pull over because he is getting sleepy a car drives by with a mattress on the roof. There is no reason to have a mattress in a world without people, the cars sleep right on the ground/floor.
Deliberate mistake: The monitor from inside Mack's trailer displays his whole face while talking to McQueen. There is no camera outside of Mack that can display him in that fashion.
Continuity mistake: After the first race, McQueen blew out both back tires. When interviewed after the race, his pit crew changes his rims with new tires, but the old rims have tires on them. They should be only rims, since earlier in the race we see him riding "on the rims" only, with sparks flying from the metal touching the track.