Continuity: While Miss Marple is chatting with the gardener and using the sprayer to kill the bugs, she generously sprays the top of the wall where the gardener's coffee cup is resting. A few moments later he drinks from it, but apparently suffers no ill effects.
Errors made by characters (possibly deliberate errors by the filmmakers): Although the character of Gwenda Reed is purportedly a New Zealander the accent is clearly Australian.