At no stage do they perform what they have rehearsed, or rehearse what they eventually perform. This "goof" is common in many backstage musicals, where the numbers shown being rehearsed rarely match the final product.
The club owner, Gerald, and Dave all mention the show being on Friday, but the Hot Metal posters they put up list the show being on Saturday, May 25.
When Dave chases Gaz out of the store much has been made of how out of shape Dave is, yet after running at full speed for quite a distance Dave is not out of breath in the close-ups.
During the dress rehearsal sequence, Dave walks past a couple walking arm-in-arm in the store. We see the same couple again, supposedly days later, in the same position and wearing the same clothes.
The factory that once employed some of the characters had been clearly abandoned for a lot longer than their unemployment had been going on for.
When the team is supposed to watch the movie Flashdance (1983), they are watching the music video of the theme song "What a Feeling" by Irene Cara instead.
When the group gets arrested at the steel mill, there is no reason why the police wouldn't let them put on their clothes first instead of getting them to the police station (almost) naked.
In the funeral scene, Lomper is playing the hymn "Abide with Me" on cornet, and his fingers are clearly visible playing the notes. He plays every note correctly until the last line, where he swaps the two notes on "[ab]-ide with [me]" - he should be playing straight down the scale Bb,A,G,F and in fact plays Bb,G,A,F.
After the arrest scene, when Gaz calls to Dave in the store the audio says, "Dave! Oi, ya deaf git," but his mouth clearly says "Dave! Oi, ya deaf twat".
When Gaz, Dave and Nathan get locked in the factory at the beginning, just after Gaz tells Dave to shut it, Nathan looks at Dave and says something, but there is no audio.
Reflection of boom mic visible in the car window as Dave throws Lomper back into the car.
Whilst the guys are watching Flashdance (1983), Dave criticizes the girl welding saying the mixture is all wrong and she's using too much acetylene. Shows how much he knows - she is actually arc welding.