Continuity mistake: Dwayne is staring at the boards after his superiors have gone to the church function. He says "JP..." and his younger coworker has his pen on the paper, but at the cut the pen hand is scratching his nose. (00:34:00)
Continuity mistake: When Florence "Ehh!"s and "Shh-shh!"s Dwayne, she does it with one hand, which becomes two at the next shot. (00:13:00)
Continuity mistake: Jack receives the photos of both Reverend Matthew Dawson and his wife; during the sequence, the black marker switches hand from right to left, to right again, and is pointing sometimes up, sometimes down. (00:11:20)
Continuity mistake: Back at the station, Jack picks Peter Baxter's profile off Florence's pile. Action happens in two shots, and the marker in his hand does a 180°; black cap up/black cap down. The mistake happens again later, with Catherine's picture. (00:10:50)
Continuity mistake: Fire exit scene. Everyone turns around surprised by Jack's appearance in the opposite direction. Florence turns twice, though. (00:07:20)
Continuity mistake: DI Jack Mooney arrives on the scene of the crime. He is walking with his left hand on his chest in the first shot, but viewed from the side, his arm hangs low. (00:05:25)
Continuity mistake: Catherine is chatting with the not-so-bad candidate, who offers his support. She says "I'd like that" with a bit of her scarf that goes in front of her shoulder, unlike the previous shot. (00:01:45)
Continuity mistake: After Judith Dawson's first testimony, Jack thanks her; Florence is just pretending to write and stops, but in the next shot she is back at it, scribbling her pad with the pen. At the end of this new shot, she stops writing, but is writing again in the next, all while DI Jack Mooney talks in continuity. (00:08:55)
Continuity mistake: At the beginning of the episode, Edwina Bousquet scoffs at the apology and gets up. Close-up of Judith Dawson, and you can see for a second in the background the bike with Danny John-Jules entering the premises. But there is no sound effect for it and in the next cuts he's nowhere to be seen; he arrives only later. (00:00:25)
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 ★