Since it is in the middle of Season 12, it is safe to say "Blue Bloods" is a solid show with a good grasp on what works.
Like all TV series, one of the main driving forces is the characters. While "Blue Bloods" succeeds at presenting strong leading characters in the Reagans with Henry (Len Cariou), Frank (Tom Selleck), Danny (Donnie Wahlberg), Erin (Bridget Moynahan), and Jamie (Will Estes), they set themselves apart with the strength of the side characters.
Among them is a tight group Frank built, consisting of Garrett Moore (Gregory Jbara), Sid Gormley (Robert Clohessy), and Abigail Baker (Abigail Hawk), who help Frank out at 1 Police Plaza.
While Moore and Gormley have clearly defined roles within the group (Moore serves as liaison to the press and Gormley having his finger on the pulse of the officers on the ground), Baker's role is less defined (per Fandom). She is Frank's primary aid, responsible for his schedule and part of his security detail — though it would be a mistake to see her as Reagan's "girl Friday." Baker isn't afraid to speak her mind on the happenings in the department and doesn't shy away from confrontation. The others highly respect her, and even though they are ready to protect her, she doesn't need it.
However, it isn't always smooth sailing with Baker. When your job requires you to challenge powerful people, there are always mistakes and missteps. Here is Baker's biggest mistake in her 12 seasons at Frank's side.
In Season 8 Episode 10, "Heavy is the Head," Baker is approached by her former sergeant, now Captain Bullman (Curt Bouril) at Internal Affairs, who offers her a position. She brings it to Garrett and Gormley (whom she affectionately refers to as "The Gs"), telling them she is hesitant. Of course, Frank finds out, and while he understands her frustration, he is bothered that she doesn't feel appreciated.
Frank visits Captain Bullman, who informs him that Baker came to him wanting to make a move. Ultimately, Frank decides to deny the transfer and keep Baker in her current position. She is understandably unhappy. This culminates in a meeting at a bar where Frank says she was being used to get a foot into 1PP. She calls him "hard and petty," but she later comes to the same conclusion. She still refuses to thank him for pulling the rug from underneath her.
While Baker avoided making the biggest mistake by leaving her position to be used by a former CO, she was blinded by the prospect of a change and allowed the entire process to transpire. Going to The Gs spread the possibility around the office and unintentionally got Frank involved when he didn't need to be.
As a cunning detective in her own right, being taken advantage of is something she wouldn't usually allow. Plus, finding out your boss was right and still calling him names seems like a brash move for her future career.