One of the characters - played by Daniel Massey - just disappears from the film without explanation. He is seen after Carol Lynley is attacked in her room but then isn't seen again, isn't murdered, isn't around at the end, and isn't referred to by anyone.
Although he is not seen, Harry (Daniel Massey) is referred to by Cicily (Olivia Hussey), who tells the others that Harry is driving her home.
When the guests arrive at the manor, it is raining cats and dogs. However, all their clothes, shoes etc. are completely dry.
Cyrus West's film apparently runs for the whole duration of the dinner that the relatives eat, with no visible changing of reels and phonograph cylinders. It is impossible for a 35 mm (or even 16 mm) film to run for this length without a change of reel, especially as the take-up reel which is visible on the projector looks to be only about 3 inches in diameter.
When two different characters are shot there is no visible damage to them. No blood, no bullet holes. Despite both wearing light coloured tops. Apparently the budget didn't run to any special effects.
The guests looked around the house to check windows and doors were locked. But they never repaired the window that Edward Fox's character jumped through.
The story takes place in 1934. It is said to be the twentieth anniversary of Cyrus West's death. Therefore the film of West shown to the prospective heirs would have been recorded, at the latest, in 1914. They did not have sound in films at this time yet West's film has sound.