The Uniform - S2-E11
Character mistake: While handcuffing an auxiliary officer in his apartment, there is a poster on the wall reading, "attention military personal..." Personal is misused. It should read "attention military personnel." (00:16:02)
Character mistake: Sergeant Gormley is promoted to Lieutenant in the previous episode. In this episode he is referred to as Sergeant Gormley by Commissioner Reagan and Deputy Commissioner Garrett Moore at least twice.
Character mistake: When Henry and Jamie are putting away leftovers from dinner, Jamie puts food in a plastic container and then puts the dirty fish in the cabinet with the clean dishes.
On the Arm - S12-E11
Character mistake: Lt. Gormley mentions a Captain (Captain Terrell) taking items On the Arm that is a 33-year veteran. Captain Terrell tells the PC that when she was 12 her brother was wounded in Iraq. If she has 33 years on The Job, there's no way she could have been 12 during the war in Iraq. The Vietnam War would have been more appropriate for the age and background of Captain Terrell. (00:00:21 - 00:14:33)
Character mistake: In this episode, Garrett says that "Nom de Guere" means pen name. It means war name. Nom de Plume means pen name and Garrett is supposed to be an expert in his field.
Character mistake: Frank implies that Joe is the oldest child, but he was born in 1977 (1x22), Erin in 1975 (6x07), and Danny 1973 (2x17). Erin also claims she wasn't born yet when the murder of Henry's friend took place in 1977 but she would have been about two years old.
Character mistake: Eddie is confronted by Captain McNichols after being caught in her office. Eddie responds by calling her Sarge, which is an incorrect title remark referring to a Captain. (00:38:46)
Character mistake: After the explosion, Danny calls for a medic. At the hospital Baez ask Danny if the reason he called for a medic instead of a bus was it because it reminded you of something that happened. Danny was a Marine, so if he was having a flashback he would have called for a corpsman instead of a medic. Medics are in the Army, not the Marine Corps. (00:06:50 - 00:54:22)
Character mistake: Commissioner Frank Reagan refers to Lt. Gormley as Sgt. Gormley in this episode. (00:08:30)
Character mistake: Throughout the episode they refer to the victim owning a "cement" company. In fact it was a concrete company. Although the terms cement and concrete often are used interchangeably, cement is actually an ingredient of concrete. Concrete is basically a mixture of aggregates and paste. The aggregates are sand and gravel or crushed stone; the paste is water and portland cement. (00:00:05 - 00:35:50)
Character mistake: Officer Rachel Witten tells Eddie the gun she has belonged to her dad when he was on The Job. In episode 15 of season 8 Gormley tells Frank that Witten's dad was a FDNY Battalion Chief and her uncles were police officers. In episode 12 of season 9 Officer Witten's dad Sgt. Scott Witten speaks to Frank about being on The Job. It's unlikely and not normal for someone to become a firefighter after being a police officer or vice versa or do both jobs.
Answer: It's a common cop show plot device. If the police on the show always acted in the most logical way (i.e. not alerting a suspect to their presence) the show wouldn't have the opportunity for an exciting foot chase.