When Victoria's body is discovered, she is on her back and the knife is in her stomach. Toward the end of the movie, Victoria is on her stomach and the knife is in her back.
When Miss Marple is taking care of Molly she goes into the bathroom. The light switches in the room are far too modern for the period that the film is set.
Miss Marple retrieves some unknown substance from Lucky's bag and hands it to Jackson, asking "Is it poison, or medicine?" Jackson tests it by dipping his finger in it and touching it to his tongue - most unwise if it turned out to be poison, and Jackson, having been in the Army Medical Corps, would know this.