Miss Marple refers to a gun that is involved in the story as a Revolver. In fact the gun is a Luger pistol which is an automatic. Automatics use one or more stationary chambers, usually in the gun hand grip, to hold bullets.
Revolvers use several chambers inside a cylinder, usually between the hand grip and the barrel, that rotates, to hold bullets.
When Bunny goes up to her room, there is a shot of a pill bottle label. It's spelled "Asprin" when the correct spelling should be "Aspirin".
The passport photo was the wrong way. It was an old British passport - the large size with a stiff navy blue cover. The personal information page was correctly shown in 'portrait' format, but the photo on the opposite page was incorrectly orientated to a 'landscape' format page. 90 degrees wrong.
When Bunner is found dead on the floor, she blinks.