Professor Margaret Winstanley attempts to convert Roman numerals into Arabic Numerals and determines that 'VXCIX' is '5,999'. Not only is it wrong, 'VXCIX' is complete gibberish. '5,999' is 'V (with a line atop it) CMXCIX.
When Barnaby is shown a plant and told that it is Hemlock, Conium maculatum, it is clearly Common Hogweed, Heracleum sphondylium.
When Mrs Plummer is looking around the garage, shortly before she discovers her husband's body, a hand that is neither his nor hers can be seen on the left side of the screen.
When Barnaby first enters the room where Madeline Villiers has just been murdered (crossing behind her), the 'corpse' blinks.