Revealing mistake: When Marnie is dreaming, watch her window. The curtains move as if there is wind, but the window is closed.
Revealing mistake: Marnie is obviously riding in front of a back-projection, as it freezes up as she approaches the wall.
Continuity mistake: At the safe, the contents, most noticeably the cash bundles, move around between shots.
Continuity mistake: About one hour into movie Marnie & Sean Connery's character are driving in a car and talking. The shot moves back and forth to the person who is speaking. Each shot shows one of them talking and if you watch behind the speaker at scenery, road, bridges, exit ramps, medians, etc., you'll notice it doesn't jive. For example there will be a grass median or an overpass out the back window when he speaks but it's not there when shot moves to her. It goes on the whole time they're in the car riding and talking.