Continuity mistake: When Atwater finds the gun in the box, it's sitting on lined notebook paper but when Voight looks in the box, the gun is now sitting on cardboard.
Plot hole: Voight and Trudy are sitting in Voight's patrol vehicle discussing Roy Walton and Mark Irwin. They state that Intelligence killed Mark Irwin. After, Voight is shown checking his vehicle for a tracker and bug, which he finds on and in the vehicle. Based on Voight's experience, he should have checked the vehicle first. Also, the case should be done, as the FBI would have heard their conversation. (00:31:03 - 00:32:05)
Plot hole: Kevin is keeping his true identity that he is a police officer from Celeste while Intelligence is investigating a murder committed by one of her art students. Although Kevin is hiding his identity from her, he participated in the daylight search of her art studio looking for the murder weapon. How was Celeste not present during the search of her studio or not aware of the search happening without Kevin being revealed as a police officer? (00:54:04)
Continuity mistake: Season 9 carries straight on from Season 8 - it is the morning after Kim's shooting. The previous night, they were wearing coats and their breath was steaming. Now there is sunshine, green leaves on the trees, and everyone is in T-shirts.
Factual error: Kevin sleeps overnight at woman's apartment. He doesn't share that he's a cop. The next morning in his unmarked police vehicle he uses his handheld radio that was appeared to be stored in the console and shows up on a violent scene with his weapon. It's obvious that Kevin used his official vehicle for personal reasons, and stored his radio and weapon in the vehicle unprotected overnight, which every police officer knows is wrong and a violation. (00:03:53 - 00:04:29)
Answer: Because he is a great cop and gets the job done better than anyone.