Other mistake: When the title card is shown, the copyright year is shown as 'MCMXXXIV' (1934). It was released in 1956 (MCMLVI).
Continuity mistake: After his meeting with Julia, Winston at home opens the drawer and in a close-up pulls the diary out from under a stack of papers which he lifted with his left hand. The angle changes, and his left hand is on the edge of the drawer, and not supporting the papers any more, nor any paper is visible. There's also a pen on the table that should have been visible before, but wasn't there. (00:36:25)
Continuity mistake: Bradford's coat collar alternates between turned up and turned down throughout the scene outside the bowling alley. (00:15:00)
Character mistake: When Henry Fonda is led to his cell, shot in a prison in Queens, a real inmate yells "What'd they get ya for, Henry?"