Character mistake: When Reid and Rossi propose the theory that Kate knew Syd and Jimmy before her disappearance, and JJ expresses her skepticism, Rossi retorts "they're handsome, charming, 19." But while reviewing the case on the flight to Maryland, JJ mentions that the boys are both 20.
What Happens at Home... - S6-E10
Character mistake: After the murder that takes place while the team is at the church meeting, they gather together to talk about narrowing down the suspect list to the people who didn't show up to the meeting. Spencer shows up with a stack of papers and says 18 suspects didn't show up. We then see his actor break character and almost laughs when he looks over at Ashley, whose actress too almost breaks character from laughing.
Character mistake: Unsub calls his victim Marcy, her real name, as opposed to Rose who he believes she is.
Chosen answer: Back in Target Rich, a hitman named Giuseppe Montolo dies whilst being interrogated by Derek. The men who abducted Derek were hired by his father, Chazz, who seems to blame Derek for his son's death. It's explained further in A Beautiful Disaster.
Annabel Keeley