Visible crew/equipment: In the scene where Henry Fonda is recreating the testimony of the old man getting out of bed & walking down a hall, you can clearly see a shadow of the camera on the floor.

Deliberate "mistake": When Henry Fonda decides to recreate the old witness's ability to get to the door in 15 seconds, he asks another juror to time his walk. The scene never cuts away, yet the juror who times it says it took 41 seconds when in reality it takes just under 30.