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.
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.
