The good brigadier, who is part of the motor vehicle theft unit, was in the office and presumably going about his duties and paperwork. He was then ordered by a superior to go and run an errand or something and stood up rather hastily to get to completing the order. Unfortunately, AU forgot that in the spirit of a gun-toting cowboy he had placed his pistol in his waistband rather than in its holster.
In his haste the pistol falls out, hits the ground, lets off a round, and shoots its good owner fair and square in the arse. There are a couple of things that are a little out of place here on the safety front, such as, "why wasn't the pistol holstered?", "why wasn't the safety on?", and "why was the pistol in the officer's waistband to start with?"
I know for sure that the last thing I would want to have happen is have my family jewels blown away by Smith & Wesson!
Apparently, the bullet has been removed and the lucky officer is recuperating in hospital.
Stupid is as stupid does.