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.
There are nine clean lines at the bottom of the pad as Miss Marple prepares her Morse message to Mr. Stringer. Yet after intercepting the shore message concerning Dusty, there are only five clean lines.
Although the training ship is supposed to be wooden and the long shots of the estuary show her to be solely a sail powered vessel, the close shots on deck (e.g., as Miss Marple is piped aboard), the ship is seen to have a funnel and steel walls.
As Breeze-Connington asks the Battledore crew to give a good impression in the wardroom, Compton puts his cigarette to his lips. The shot changes, and he lifts the cigarette again.
On entering the grounds of the house to be burgled, the petty officer carefully closes and latches the gate behind him. Mr. Stringer, following close behind, pulls open an unlatched gate.
The 'messages' sent by both Miss Marple and Mr. Stringer bear no resemblance to Morse code and are just random flashes of light. The actual messages sent at approximately the speed of the flashes would have taken over a minute, which is presumably too long in movie time.
When one of the trustees gets poisoned with strychnine in his snuff, he dies instantly. In reality, the dose of strychnine required to kill someone is much too large to be contained in a pinch of snuff--the LD50 (lethal dose for which 50% of the population would be expected to perish) is 16 mg/kg; so, in order to kill an average person (70 kg/154 lb), the murderer would have to administer more than a gram of strychnine; close to two grams to be sure. In the past, strychnine was used as a recreational drug and an athletic performance enhancer. Also, death by strychnine overdose is not instantaneous; it's caused by a series of convulsions of increasing severity until the victim is unable to breathe - a slow, lingering death that is very nasty.
Commander Breeze-Connington's conducting of the chorus is inconsistent, but the singers aren't following it anyway. His downbeat is never where it's supposed to be, but the singers maintain the proper tempo throughout the song.
Dr. Crump says that Curare is a poison favored by the Pygmies of South America. Curare is only available in South America but the Pygmies are primarily from Africa.
The brisk Dr. Crump says that the police are "just across the road" from the Trust's meeting. Although this seems incorrect as the County Constabulary HQ is sited next door to Milchester House where the meeting took place, there is actually a road between the two buildings. It is, however, more likely to be described as a lane than a road.
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.
Charles 'Bud' Tingwell's character is wrongly credited as "Det. Insp. Craddock" - as the dialogue makes clear, he has been promoted to Chief Inspector following the events of Murder Most Foul (1964).
The typeface of Jim Stringer's first name is reversed in the closing credits, spelled MIJ with the J backward. This does not occur on the Australian DVD where he is credited as "Mr. Stringer".
The character played by Gerald Cross is introduced to Miss Marple as "Lieutenant Commander Dimchurch", but is credited as "Brewer" in the end credits.
During the opening credits when Miss Marple is dressing up as a sailor she obviously has to change her clothes she came into the shop in. When she has completed this and looks like a sailor she leaves the shop. She does not leave carrying the original clothes she came in with.
When Miss Marple is being piped aboard the Battledore, the side party don't move the fingers holding their bosun's pipes as they should have done were they actually using them.