Other mistake: Clarence tells George that his brother died at the age of 9 because he wasn't there to save him from falling through the ice, but when you see the tomb stone, it shows Harry Bailey was born in 1911 and died in 1919. That would make Harry only 7 or 8 years old.
Continuity mistake: Watch Devlin's hands in the cafe in Rio. They open and close between shots.
Revealing mistake: When Ballin's plane flies and finally crashes, it becomes obvious that the horizon is a still image with the film of the explosion superimposed directly onto it.
Visible crew/equipment: When Fred kisses Peggy, a boom mic is reflected in the left side of the car.