During the party, Shaitana talks to a waiter and pushes him towards Poirot and Mrs. Oliver. When Shaitana pushes him, there is one cocktail glass on the tray carried by the waiter. When he approaches Poirot and Mrs. Oliver, there are two.
Shaitana uses what appears to be a gas cigarette lighter. Poirot is set in the 1930s and gas lighters were invented in the 1940s.
When Major Despard is talking to Millie in her cottage, a modern burglar alarm motion sensor is visible over the interior door.