Continuity mistake: The lads have been caught by the police during the dress rehearsal. Their names are being taken and the clock on the wall in the police station says 11.40 a.m. - it's in daylight. The security videos are brought in and the time on the one they watch is 12:27.
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.
Continuity mistake: When Gaz is trying to cadge £100 of his ex wife at her factory her head is tilted one way on the long shot and the opposite way in the close ups.
Plot hole: When Gaz tries to steal a jacket and Dave, who works as security, catches him in quite a dramatic manner they are seen by other employees. Dave lets Gaz go because he is a friend. But the next day both stroll together through the shop as if nothing had happened, with no enquiries or follow-up from the other employees.
Plot hole: When Gaz, Dave, and Nathe are trying to cross the canal with two stolen girders via the semi-sunk car one girder is balancing on the car roof and the other one lies between the hood and the other side. Nathe walks across the girder, Gaz asks him to pull it onto the land, and Nathe lets it drop into the water. This doesn't make sense for two reasons: 1. It's obvious that the girder is too heavy for Nathe, 2. They would have first used this girder to carry the other one across, which must also have been the way how they reached the car in the first place. The movie depicts Gaz not exactly as perfect but still too smart for such blunders.