The scratch that Captain Reilly's wife gave him on his right cheek disappears in the next scene.
When Chief Jerry is talking to Chef Perolli about his alleged "illegitimate child", the scratches on the right side of Jerry's face that he got earlier from his wife are gone, and his skin is clear. In the next scene, the scratches on his cheek and nose are back.
Lt. Delfino is shown wearing twin bars on his collar, indicating the rank of Captain. He should have only one bar on each pin.
The scenes in season 2 that take place at the Staten Island firehouse were filmed in the quarters of Company 2 of the Manhasset-Lakeville Fire Department, a department just outside Queens in Nassau County. The indications of this are visible on the truck, since though the words "Manhasset-Lakeville Fire Deparment" were removed, the gold leaf striping that does not appear on FDNY vehicles is still visible, as well as the words "Company 2" and the truck's M-LFD designation, "8728." Furthermore, if you look closely at the far wall in the shots of Denis Leary while he is discussing the singing competition with the other guys, you can make out the letter's "M-LFD" on the bunker jackets hanging there.