Deliberate mistake: In the very first scene, with Dave, Gaz and Nathan in the abandoned steel works, they only have one girder, but when they're on the car in the middle of the canal, they have two: one balancing on the car roof, and the one Nathe walks across.
Suggested correction: There's been a time skip between the two scenes. They found another girder and took that one as well.
Continuity mistake: In the opening scene when the Works band come round the corner, watched by Gaz, Nathan and Dave who are hiding, you see Limper at the back of the band, playing and marching with them. His ginger hair is easily recognisable. Seconds later he's looking up as the security guard. It seems unlikely he'd put his instrument down and leave the band to come and lock the doors. Of course, he appears again in the band later in the film, so we know it was him in the first shot too.
Suggested correction: That's exactly what happened. Lomper quickly ducked out of the band because he realised he'd forgot to lock up. You can see him holding his instrument while he does so. The whole point of this part is to establish that Lomper is a member of the band and works there as a security guard. As such he can provide the gang somewhere to rehearse.
