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".
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.
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.
Characters are seen smoking King size cigarettes.
Not available in the United Kingdom until the 1960's.
When Bunner is found dead on the floor, she blinks.