When Lucy enters the dining room with water after the family have been poisoned, she's wearing a dark skirt and sparkly sweater with a patent leather belt. When she is walking up the stairs right afterwards and bumps into Dr. Quimper she's suddenly wearing a black button through dress with red patent leather belt. There's no logical reason why she would have gotten changed.
When Lucy is checking some documents, she looks through a passport. The photo has clearly simply been stuck onto the page and there is no official stamp overprinted on it.
(around 9min) the newspaper has backward print.
Emma Crackenthorpe has lipstick on her teeth during the train ride, hearing about how the killer did it.