When the valuable painting at the center of the intrigue was first introduced it appeared that the picture wasn't very large. However when Stringer sets off for London to have it appraised, it looked like it grew to twice the size.
When Miss Marple and Stringer enter her cottage, they go through the front door which has a window on the same level as the top "Yale" lock. The next scene inside the cottage shows the door without a window.
At the second murder scene, Miss Marple grasps Inspector Craddock's arm to tell him something. When rebuffed by him, she is shown removing her hand twice between shots.
After Hector is thrown by Black Jack, he rides back to the stable on Miss Marple's horse. He gets off the horse and puts his full weight on his left foot and brings his right leg down gingerly. Afterward, he favors his left foot, and it is the focus of all the pain.
Miss Marple is awakened by her alarm clock at 10:15 p.m. Later that evening, after her 'spying' escapade and she is back in the hotel, a clock on the wall shows the time as 9:30.
From the outside, Miss Marple's cottage has a multi-pane window just to the left of the entry door, but when she and Stringer go inside, there's a solid wall with a coat rack hanging on it.
As was customary film practice at the time, the lightbeam from Crossfield's torch in Black Jack's stable was provided by an actual studio light rather than the actor's prop. This is obvious as the light spot on the wall remains fairly constant whilst the character moves frantically - and at one point the torch casts a clear shadow in its own light.
When Miss Marple first opens an upstairs door after Mr. Enderby's death, she briefly sees a cat perched on top of a chest or some sort of high furniture. The 'live' cat is obviously a stuffed prop when Miss Marple first sees it, and in the next cut the cat leaps from the chest and runs past Miss Marple.
In one scene, Rosamund and Michael are riding in a car down a dirt/gravel road. When Rosamund slams on the brakes to stop the car, the sound effect is of the tires screeching, which would happen only on asphalt or concrete.
When Inspector Craddock first interrogates the whole family in the main room, you can just see him taking a big step over a carpet-covered Dolley track, then the camera rolls in and the covered track is not visible.