At the beginning of the film the lesbian and gay ring a miner's club to offer support. The phone at the club rings in an American tone and not a UK tone.
The button Mark gives Joe when they meet after watching the miners returning to work in Onllwyn says "I am (discretely) gay". This is a common error, but it should be "discreetly" meaning cautiously or with tact; "discrete" means separate or distinct. However, it could be a copy of a vintage button, in which case the error is on the part of the original designer of the button, not the filmmakers.
When the first phone call is made to the Welfare Hall, we hear a single US-style repeated ring instead of what would have been heard at the time: the UK double-ring followed by a pause.
On Joe's birthday, in July 1984, a record of 'What Difference Does It Make?' by The Smiths can be heard. Whilst the original song was released in 1983, the version playing in the film was one recorded for the BBC, which was not actually released until November 1984.
When Mark Ashton proposes the founding of LGSM, he holds up a newspaper with the front page showing an iconic photograph of a mounted policeman about to strike a woman. This was taken by John Harris at the so-called "Battle of Orgreave". Although subsequently famous, the photograph was still obscure in June 1984, when the scene is set.
In the first aerial shot of the bus crossing the Severn Bridge, the bus is moving but the waters are perfectly still.
The scene where the activists are ignorant of the locations of the coal mines seems very unlikely, given that the names and locations of mines were mentioned on national news on a daily basis during the strike, and given that the Kent NUM had been raising funds in London from the start of the strike.
In the opening scenes we see a speech by Arthur Scargill being shown on a portable TV set. The picture is in colour but that was a monochrome TV, not a colour one.
In a scene set in 1984, Joe and his family are seen watching the government TV advert "AIDS: Don't die of ignorance". The slogan was first used in November 1986 and the advert was first shown in January 1987.
When 'Bromley' is having photographs printed they are printed on an inkjet machine. In the 1980s all photographs would have been printed on photographic paper.
The Laura Ashley wallpaper the women admire on their trip to London is from the 1987 catalogue and wasn't available in 1984.
In the dance club, set in mid-1984, the song "You Spin Me Round" by Dead or Alive is playing. This was not released until the very end of 1984 and not a hit in clubs until well into 1985.
There is a shot of vehicles driving along a 4-lane motorway, presumably the M4. There were no 4-lane motorways in 1984.
When the LGSM group return to the village in winter and we see a CGI'd image of the village with snow, several shots later deciduous trees in full green leaf can be seen in the background.