Revealing mistakes: 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.
Boom mic visible: Reflection of boom mike visible in the car window as Dave throws Lomper back into the car.
Continuity: 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.
Plot holes: 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.
Errors made by characters (possibly deliberate errors by the filmmakers): Whilst the guys are watching Flash Dance, 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.