In the talk-show scene before Sir Miles Alexrod comes on screen, a car which appears to be Lightning McQueen very briefly appears on the WGP graphics. The incorrect car is used, as the "trails" seem to correspond to the car's country, and the trails with Spanish flag colors (red and yellow) have what seems to be Lightning McQueen on them.
As Lightning & Mater are heading back into town after Mater's day of fun, Mater is on the left and Lighning on the right. Lightning drives in front of Mater to tell him he's having a quiet dinner date with Sally and when we cut back to Mater he's now on the right of the road.
When the cars race from Trafalgar Square to the Mall, they go through Admiralty Arch. All three main arches are open. In reality the central arch is always closed to traffic except on state occasions when the Royal vehicles are allowed through that way. Normal traffic only uses the outer two main arches.
When the race tracks are shown for the Japanese, Italian and British races, the Union Jack is shown with all the white lines being the same width. In fact the red diagonal cross is offset, so that on each diagonal, the white lines each side of the red are different widths.
Brent Mustangburger has clearly visible white walls on both the inside and outside of his tires. Tires are manufactured with a white wall on only one side of the tire so that the user can decide whether or not to have them visible when installed on the vehicle.
The goof items below may give away important plot points.
When Tow Mater is Knighted, he would not be referred to as "Sir" either by the Queen or on the billboard. Only citizens of countries recognizing Queen Elizabeth II as their head of state (e.g. UK, Canada and Australia) may use this title. Citizens of other nations may never be called "Sir" when receiving an honorary knighthood.
When Mater, Finn, and Holley are tied down on Big Bentley's gears, ready to be crushed when the gears run into each other. It is established that the gears rotate for every minute that passes on the clock. When the clock reaches the time of 3:02 they are all two minutes away from being crushed. The scene cuts away and when it returns, the clock face clearly shows a time of 3:05, but the gears have not moved inside.