Stupidity: 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. (00:59:30)
Stupidity: 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. (00:03:40)
Answer: It's a sight gag. Dave's essentially stealing the candy, so the alarm goes off. When he crushes it, the alarm stops. It wouldn't really happen, but it's funny that the alarm stops when he removes the candy.
ChiChi