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.
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Suggested correction: There's been a time skip between the two scenes. They found another girder and took that one as well.
Deliberate mistake: Directly after Nicola has her baby, she sits up and holds a baby that is clean, umbilical cord-free, and has teeth. There is no possible way this could happen.