During his exchange with Bunter in front of the fireplace, Puffett's left hand goes from resting on his hip to prodding the chimney instantaneously, with the stick he prods with appearing out of thin air.
When Peter examines his new bracelet, it changes its appearance between shots.
After Bunter announces a small inconvenience in the kitchen during the Wimseys' dinner and leaves, Harriet's hand changes position instantaneously.
An American cannot be an elected peer in the House of Lords such as is the lead actor in this film.
When the cat flees to the basement, the basement door mysteriously opens for the cat to enter. Nobody or physical reason for the opening.
The housekeeper at Tallboys tells Lord Peter that Aggie lives at the "last house in the village, right-hand side." When Peter and Harriet pull up to the house, it's on the left.