Plot hole: Much like the 1978 movie adaptation, in this version there is an absurdly short time elapsing between Linnet leaving the bridge table and the incident and murder; it's barely two minutes when she'd have to go to her cabin, do everything a proper lady of the time would do to prepare to go to bed, and fall sound asleep. That's because the witness character (Cornelia Robson here) in the novel is supposed to be droning about her boring life for a long time, but there are no gaps in her speech to imply that a long time passed - at most there's the waiter bringing a drink at the very beginning who sorta comes out of the blue.
The Tragedy at Marsdon Manor - S3-E6
Plot hole: It is revealed towards the end of the episode that the painting Mrs. Maltravers did when the murder took place was not painted that day and that time of the day, because it had the wrong shadows. But we actually saw what was on the canvas when the secretary was leaving to go to the bank, and it was that exact painting in an early state. She also came up with the idea for the murder the night before, making even more unbelievable that she'd just have the alibi painting ready and waiting the morning after. That without mentioning the fact that the murder itself as described is not something that would have required an amount of time needed to make a whole painting, and that she could have finished it later anyway (she gave it to Hastings a day later, after all).