Revealing mistake: When Lightning McQueen is being shown the simulator in the plant, the times shown on the simulator don't work. A regular timer would say, for example, 21.99 seconds, but the timer on the simulator screen only goes up to 21.27 seconds, if you watch it. I assume this is because of the Frame Per Second of the movie, and wasn't noticed, being as most cartoons etc run at 25-30 FPS.
Factual error: In the end credits, some of the vintage advertisements have 770 area code phone numbers, which while valid for Atlanta, Georgia and surrounding areas now, it didn't exist until 1995 when it was assigned. The numbers should have had either a 404 or 912 area code since those 2 area codes covered all of Georgia until 1992 when area code 706 was implemented.
Continuity mistake: During the Los Angeles 500, the Next-gen Carbon Cyber racer Chris Roamin' can be seen behind McQueen. However, when McQueen crashes, Roamin' is nowhere to be seen and instead the Next-gen Nitroade racer Tim Treadless and Next-gen Easy Idle racer Harvey Rodcap are behind McQueen.