The brisk Dr Crump says that the police are "just across the road" from the Trust's meeting. In fact, as has already been established, the County Constabulary HQ is sited next door to Milchester House where the meeting took place.
Miss Marple supposedly searches through the cabin of Lt. Compton, when she finds the suspicious envelope. However, the door sign reads "Capt. Rhumstone". When we take into account that each officer moved into the cabin of the next lower ranking officer, the Captain now lives in the cabin of Commander Breeze-Connington, and that would be the cabin Miss Marple is searching through. Moreover, when moving into the new cabins, all the officers' cabins appear to be located in one single row. Later in the movie, however, they appear to be quite irregularly spread around the deck.
Although the training ship is supposed to wooden, and the long shots of the estuary show her as being solely a sail powered vessel, when we see the close shots on deck - for example, as Miss Marple is piped aboard - we now seem to have a funnel and steel walls.
As Capt Rhumstone prepares to vacate his quarters he places two liquor bottles on top of the cabinet with the labels facing towards us. Minutes later, when he makes for the door, the labels are facing away from us.
As a small boat approaches Battledore Miss Marple is seen peering out of a cabin window which overlooks the ship's gangway. The window frame is painted all black, yet when she is seen again at the same window the inner frame is painted white.
Behind the opening credits Miss Marple is in a shop changing room and being dressed with clothes successively removed from a shop dummy. However the clothes are obviously removed from the dummy in the reverse order from that in which they would be put on.