While shooting the "flying dog" scene, the sky changes from cloudy to blue and back almost immediately. That happens often in Britain, but not *that* fast.
When Will is gets the stew pot out of the oven, he places it on the table using a cloth with two pockets. In the next shot, from behind, he takes the lid off with his bare hand. In the next shot, from the front, he holds the lid with a cloth.
When Didier Revol demonstrates his acting to Will Proudfoot in the church, a cigarette is in Didier's left hand. When he "shoots" himself in the head and falls to the ground, the cigarette vanishes. The smoke is in the foreground in one shot, but Didier never put it down.
Carter films First Blood (1982) with a camcorder at the cinema, but the aspect ratio is 1:85:1. "First Blood" was a CinemaScope film, so that aspect ratio is incorrect.
When Will watches the pirated version of First Blood (1982), the cinema curtains open on the film's opening credits. In 1983, all films screened in British cinemas started with the British Board of Film Classification's certificate just before the start of the film. If Lee started recording after the certificate was displayed, the curtains would've been open when the opening credits began.
The brick wall at school shakes when Didier's entourage shakes Lee against it. (According to director Garth Jennings at 58:10 in the DVD commentary, it was a fake wall built by the production when the screenplay called for action in a narrow corridor.)
Despite a clear lack of wind, the kite stays in the air during the "Flying Dog" scene.
Siouxsie And The Banshees' "Peek-A-Boo", which was released in 1988, plays at a school party.
The First Blood marquee in the opening shot says the film has a "15" rating. That certificate was introduced in 1984-85, part of the Video Recordings Act. In reality, the film was rated "AA," meaning "Passed as suitable only for exhibition to persons of fourteen years and over. When a programme includes an 'AA' film, no persons under fourteen years can be admitted."
When Carter is waiting at a gate, the cow behind him has a yellow id tag in its left ear. ID tags for cows were introduced in the 1990s, after the BSE (Mad Cow) crisis.
Didier Revol rides a skateboard that was first sold in England in the early 1990s.
The bus that leaves with the French exchange students has a yellow and blue European Union license plate on the rear. The European Union did not exist in 1983.