When Mr. Folger looks through his binoculars at the twins in their storm-tossed sailboat, the view is one of fair weather.
Admittedly the special effects/trick photography are superb in this film, especially for its time, there is a moment, just after Kate hands Pat a lit match, when Kate turns transparent. It's when she's behind the chair Pat's sitting in, and moves off to our right. As she starts her move, her waist becomes transparent for just a half second, and we can see the bed behind her through her hip and waist area.
When Katie and Pat are talking in the bedroom after the barn dance, Pat tells her that Bill said she's "swell and they had a lot of fun together". Katie walks away from her and in that shot only, it's obvious by Pat's face that it's a stand-in and not Bette Davis herself.
During the storm on the boat with Kate and Pat, the sail is briefly visible in the shot. The sail shows a CF registry number which is a California registry when this story takes place in North Eastern United States.