Episode #3.4 - S3-E4
Other mistake: Twice in the episode Fidel gets from the computer a fingerprint match for Adam Frost. The date of the sample is February 10th, which not only is inconsistent with the supposed timeline of the episode (the security camera puts the murder on March 7th, and the previous episode was set in the fourth week of March as per the gigolo's appointment schedule). but also differs from Natasha's fingerprint date, February 9th. The murder was discovered in the morning, so the victim's prints have been taken the same day as the suspects'.
Episode #3.7 - S3-E7
Other mistake: Camille is the fastest reader ever; she holds the newspaper for TWO seconds and already she can tell that Valerie's dad lost his business because of Jackson (which is something the article itself, if you check it out on freeze-frame, does not say, incidentally - even if it did, she couldn't extrapolate the information this quickly from the page). (00:33:15)
Episode #3.8 - S3-E8
Other mistake: The timestamps on the CCTV screenshots indicate that the murder happened in early March (since they are from early February and they go back one month), but March already passed in previous episodes of the same season (gigolo calendar, bank statements). (00:29:00)
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 #4.4 - S4-E4
Other mistake: On the first day of investigations, Florence says that "two days ago" the victim withdrew a large sum of money. A CCTV shot is shown later in the episode, establishing that day as March 9th 2015 - remarkably consistent with the date of the previous episode and the coroner's report. So it is the 11th, and the investigation concludes on the 12th, a Thursday. However, at the end of the episode, Dwayne says that it's Saturday.
Episode #4.7 - S4-E7
Other mistake: Spoiler: While unmasking the culprit, Goodman never explains what happened to the real Dominic Claydon. Only that the perpetrator used his identity to get what he wanted.
Other mistake: Commissioner Selwyn Patterson puts some pressure on Humphrey and the team saying that "a wealthy tourist has just been killed." But Dan Hagen was not a tourist; he lived on Saint Marie with his wife, as said by the fellow oceanographer Jonathan Taylor, and they have a villa. Someone as well connected and nosy as Patterson wouldn't miss the notion of a multimillionaire living on the island. He also says that he needs to reassure the yacht club that there are no pirates in the area, but it was already established by the coast guard that no ships approached the area. (00:12:15)
Other mistake: It's played as a gag, but still it's quite inconceivable that in "fifteen minutes" it would get pitch dark so that Dwayne can avoid his duties sanding the ship! How much time could he have bought even if he had more excuses up his sleeve? Goodman really can't tell the difference between 15 minutes and a couple of hours? The scene started when it was nowhere near dawn. (00:21:10)
Other mistake: Florence closes her notepad and puts the cap back on the pen when Dwayne just begun telling everyone what the Gendarmerie reported about one of the suspects. She has no way to know by that point what Dwayne is about to disclose, and that Humphrey is about to leave with her. (00:17:05)
Other mistake: Goodman receives the package from the postman, and it is stated that Cedrik bought the watch on the 10th, which is said to be 2 days before his death. His death being on the 12th is inconsistent with both Dwayne's phone desktop (July the 4th) and the message (Thursday 29th). (00:25:20)
Other mistake: Humphrey's flashback when he is on the balcony of the room is about a sentence he never heard, since the second interview was done by Florence and Dwayne.
Other mistake: Dwayne goes through the deleted text messages in Robert's phone. The messages have as dates "Thursday 19" and "Saturday 21st." Given that the year is 2016, the dates are compatible with the month of May, but the episode is set on April 29 - it's the date on the police report for both Matt Holt's background check and the bank receipt Gary Holt shows. (00:19:25 - 00:29:45)
Other mistake: JP tells the others that his fiancée cooked him a great meal for the two-month anniversary the other night. Suddenly, the three months anniversary is coming up and he proposes - but just a day passed.
Other mistake: It is never clarified why the victim paid his brother; besides that, the bank statement has the date of April 29 and most facts point at the date being the one the episode is supposed to take place at. The investigative report though refers to a meeting happening in January - would hardly be something the aggressive legal team of the even more aggressive chef would keep on their desk for 3 months.
Other mistake: In Naomi Walker's flashback when she gives Goodman her recounting of the evening, she gets the call from her husband when the phone is on the table - she got it with the phone in mid-air as she was standing already, instead. (00:11:10)
Other mistake: The police force is searching the room of the deceased at the beginning of their investigation, Humphrey stands in front of a map of Saint Marie with the exclusion zones. The map though shows an island upside down compared to the map you could see in Episode 7 of season 2. (00:10:10)
Other mistake: In the bank statement that Dwayne hands to the DI, there are cash withdrawals "from the past three months", with the past three months being January, February and...February, again. (00:20:30)
Other mistake: Torey Martin says that he had an accident 5 months before, but the car crash evaluation is signed 31/07/2016. Not only it was a Sunday - unlikely day for the car shop to be working -, but that would put the events of the episode on January, contradicting the bank statements and what was established in previous episodes.
Other mistake: The police boat approaches the scenes of the crime coming in from the right, in a straight line, then in the proximity of the boat they turn around it, moving clockwise, passing in front of the bow and then to the left side. When they review the readings though, the icons show a completely different story, as if the police boat approached from behind and they boarded docking right at the stern. (00:04:40 - 00:08:00)
Other mistake: Days in the series seem sometime to go on for unusually long amounts of time. In this episode, there's the exact opposite; the murder happens at 10 AM (which already seems awfully late for the opening of an election), the police obviously are called right away, Jack and Florence go to the station to discuss the case, and then all of a sudden it's a wrap when Dwayne and JP get back there with their findings. Unless they have searched the businessman's residence for over 6 hours while Jack and Florence have been doing next to nothing at the station, it makes no sense.
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.
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