The cost/gallons numbers on the gas pumps change several times during the gas station race set-up scene.
At the garage where the race is being set up, watch the crank of the hand mangle as it mysteriously changes its position throughout the clips.
In one major scene the Mechanic (Wilson) scopes a drive in parking lot for a competitor. He notes "a '32 Ford Coupe with a 427 and 11" rubber". The car is in fact a '30-'31 Model A Ford with a GM radiator grille. The rubber and Chevy engine appear to be correct, though.
The interior of the GTO changes between chocolate brown and black.
The wheels on the GTO change from GM factory rally wheels to Keystone mags.
Early in the film when they enter the small town they have $300 for racing bets which they use and double to $600. But later in the film they have no money (bread) but couldn't possibly have spend that $600 on food, petrol (gas) and a single room for a single night in a motel.