When the fight starts between Stewart and Downey at the beach, Downey's hat comes off revealing a much younger stunt double with a thick head of very dark hair. The older John Hoyt, portraying Downey, appears after a cut again with short, graying hair.
Although realistic looking $20 bills are seen individually as both the real thing and as counterfeit bills, when the moment of truth arrives and the counterfeit inventory is handed over, the stash is obviously stage money.
When Sylvester is being chased through the trolley barn, the pistol he is using changes from a revolver to a semi-automatic.
When Sylvester is hiding from the cops in a trolley car and accidentally steps on a button that rings the bell, the cameras speed up and the cops rush off as if they're in a Keystone Cops comedy, blowing all verité.
Outside the police station where John Hoyt bailed out Barbara Payton, a clear reflection of the cameraman in action can be seen between the actors.