Remembering from film school...
Aparently Citizen Kane used alot of Mattes and double exposures, each with different focus points set. The famous shot with Welles in bed, the glass of water in the foreground and the door in the background was shot on 3 separate passes, first the glass, then Welles, then the door. I heard somewhere that no one really knows how often this technique was used, but some have estimated that as much as 40% of the shots in Citizen Kane involve some sort of visual effect, mostly done in camera.