While the track announcements are given in both French and English at the beginning of the race (and movie), throughout the rest of the movie they are in English only, which would not occur at Le Mans.
Near the end, Wilson (blue Porsche) is being chased by Delaney (blue Porsche) and Stahler (red Ferrari) who are side by side - however another shot just afterwards shows Wilson being chased by 2 red cars that are side by side.
Levels of dirt on Delaney's Porsche during the final scenes.
During the final lap of the race, Stahler's Ferrari kicks up considerable dust as he grazes the track edges after rounding Mulsanne and Arnage. However, it had been raining heavily earlier in the day, so the ground should still be damp.
During a night pit stop, you can see the breaths of McQueen and his manager, even though the race takes place in June. This may have been due to the movie shoot running well past schedule, ending in November.
The 911 that Steve McQueen's character is driving around Le Mans and the racecourse in the beginning has a Stuttgart license plate (home of the Porsche company). However, the car is the type equipped for the U.S. market (front turn signals that act as driving lights, different headlights, red rear turn signals), and the license plate has no seal from the registration office.
As Delaney and Stahler duel for position late in the race, their car headlights turn on and off randomly with changes in camera angle, sometimes seemingly in mid-corner.
During the final laps, car number 21 can clearly be seen without a driver.
In the scene describing the accident involving Delaney and Belgetti, the man talking with Belgetti's wife speaks Italian but pronounces her name with a hard "g", as an English-speaking person would.
The main accident scene has at least four cars involved, but only the important two are mentioned by the track announcer.