Plot hole: The plot resolution hinges on the fact that the culprit gave the illusion of the door being locked while the lock was already broken. And he did that by jamming under the door a rusty old fork, through the usage of a string. However, this appears really far-fetched to say the least; if the door was obstructed by an object under it, it would have not given way as if the lock was busted. It would have dragged, created a screeching noise and scraping the floor leaving visible marks (which considered it's a locked room mystery, would have been investigated). The person breaking in was Jack, even, who is supposed to be really perceptive and crime-savvy and not the average person.
Death in Paradise (2011)
1 plot hole in Melodies of Murder
Continuity mistake: When Aunt Mary is writing on the whiteboard at the hospital, the word "killed" changes style between shots. (00:31:40)
DI Jack Mooney: My grandfather told me a story once, about these two wolves fighting inside all of us. And one of them is anger, envy, self-pity, regret. The other one is love, truth, faith, hope, that sort of thing.
And I asked him "which one of the wolves would win?" And do you know what he said?
"The one you feed." (00:18:15)
More trivia for Death in Paradise
Question: Why did the cord on the wall phone in the police station change from straight in the early episodes to coiled later?
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 ★
As long as such changes aren't an obvious continuity mistake (e.g. changing multiple times during a day), one can just assume the change happened in-universe as well. Things do get broken or updated occasionally. It adds a bit of realism, in my opinion.