Other mistake: Joséphine Jobert is a Sergeant (the commissioner introduces her as such at the end of the first episode), but in the end credits of all the episodes before she takes over for Camille Bordey and becomes officially a Detective Sergeant, she is merely credited as "Officer Florence Cassell." By comparison, Fidel was credited as "Sergeant Fidel Best", even not being a DS.
Episode #4.1 - S4-E1
Continuity mistake: Lucy Thomson sits at a table outside while her brother Daniel is dealing with distillery paperwork. She puts her cell phone next to her. When it rings a few shots later, the phone is at the corner of that table. (00:15:45)
Episode #4.1 - S4-E1
Continuity mistake: Camille and Humphrey pass to each other and show Andre Morgan the key retrieved from the safe. They hold it positioning their fingers differently (tip of the key or closer to the rounded part) on the various shots. (00:18:50)
Episode #4.1 - S4-E1
Continuity mistake: When Dwayne speaks with the mysterious woman at Catherine's, her hair looks different in close-ups and wider angles. (00:19:50)
Episode #4.1 - S4-E1
Continuity mistake: Dwayne realises that he did not ask the cute girl's name, and looks at her now far away. Something is wrong with this shot of Joséphine Jobert and her derriere; walking down the same street there's the same couple that is having a drink right next to Dwayne (his black shirt with colourful drawings is not exactly hard to miss). (00:20:50)
Episode #4.1 - S4-E1
Continuity mistake: When Humph realises he got wrong the timing of the jumpscare in "The Ghoul" with Boris Karloff, Camille when it cuts to a wider view is holding her popcorn at lower height. (00:23:40)
Episode #4.1 - S4-E1
Continuity mistake: When Camille pummels her boss with a pillow, she is holding it with the striped side facing the camera. In the close-up where she puts the pillow down, it's the black side. (00:23:50)
Episode #4.1 - S4-E1
Continuity mistake: Humph and Camille are talking with Zeta Akande about the reasons for the lack of references from her job. When she says "The children really loved me", her braids at the leftmost side of the chest are bunched together, split when she says in close-up "He didn't like that", united again as it gets back to a wider angle, etc. (00:32:35)
Episode #4.1 - S4-E1
Continuity mistake: The bottles of beer in the hand of both Humphrey and Camille are turned facing different directions in each shot in their final scene together. (00:49:40)
Episode #4.1 - S4-E1
Continuity mistake: When Humphrey sets the popcorn for Harry, Harry in the last shot is closer than in the previous - which could make sense if we assume he scuttled closer during the split cut to Kris Marshall, but the light on the couch changes between shots as well. (00:51:15)
Episode #4.2 - S4-E2
Continuity mistake: The future victim is preparing the client for some surfing. During the scene, the strap of the surf is in the sand in Jake's closeups (you can see it popping up in the left part of the screen) but it's at the back of the board in the wider angles. (00:00:25)
Episode #4.2 - S4-E2
Continuity mistake: Officer Florence Cassell is bagging something behind Dwayne as he is talking about the case. When he says "It's what the evidence says, Chief", she goes all of a sudden from holding the ziplock bag to holding the notepad. (00:07:55)
Episode #4.2 - S4-E2
Continuity mistake: When Goodman climbs on top of the furniture leaving his colleagues puzzled, his pen is simultaneously in the hand that grips the crack in the wall net to the ceiling, and in the hand down his side together with his notes. (00:08:15)
Episode #4.2 - S4-E2
Continuity mistake: During Dwayne's hypothetical rendition of the crime, Florence is writing with the notepad's cover held up, except in one shot ("And then goes out of the back window"), when she has it rolled against its back. (00:08:55)
Episode #4.2 - S4-E2
Continuity mistake: After Dwayne asks if it's OK to release the body and his boss agrees, Florence turns around to randomly photograph things. She does it with both hands, but just one hand in the next shot. (00:09:15)
Episode #4.2 - S4-E2
Continuity mistake: Goodman and Bordey are done with the first round of questions to the school members. As Camille begins to walk out, Charlie Beckett is sitting with his hands between his legs, but in the next shot he's sitting with his hands behind his back. (00:11:50)
Episode #4.2 - S4-E2
Continuity mistake: When Humphrey knocks over the cup of coffee, the opening of the cup faces away from the table lamp. When he pulls the sock out of his jacket, the cup is facing the other way. (00:15:10)
Episode #4.2 - S4-E2
Continuity mistake: When Charlie Beckett sprints away, seen from behind in a faraway shot, the towel from one shoulder runs down his back, but in the next shot he still has it draped around his neck. (00:18:20)
Episode #4.2 - S4-E2
Continuity mistake: Florence just mentioned the shocking truth about Jess Chambers - Cornwall has a surf club! Humphrey turns around ordering to dig through the club finances - but in the first shot he is pointing his right hand, and in the second he's doing it with his left. Even more obvious a few moments later when he says "Hang on!"; points the finger, but with the right hand, then left hand. (00:21:00)
Episode #4.2 - S4-E2
Revealing mistake: Camille puts the name of the victim and "Watergate" as search terms, and manages to find the relevant news item from the past. The URL of the website though is just the one of the (fictitious) homepage (watergatenews.org), and not of a specific article. Moreover, the article is half relevant info, and half generic guide about surfing with a brisk cut from one to the other. (00:21:50)
Answer: There's probably no particular reason. Sets and props on long-running TV shows often change as needed and for various reasons throughout a series run.
raywest ★