Deliberate mistake: When Henry Fonda decides to recreate the old witness' ability to get to the door in 15 seconds, he asks another juror to time his walk. The scene never cuts away, but the juror who times it says it took 41 seconds when in reality it takes exactly 31 seconds, you can time it.
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.
Continuity mistake: Juror #4 announces the time to be 6:15, a couple of minutes later when Juror #3 is about to break down, tearing up the photo of himself with his son, his watch shows 5:10.
Visible crew/equipment: When Juror 8 is approaching Juror 4 following the revelation of the witness across the street's eyesight being put under question, as the camera moves in, its shadow is seen moving across the back of Juror 3. (01:28:55)
Answer: He was actually talking about the night before the last one. Trials, especially longer ones for serious crimes such as murder, are not usually on consecutive days...often there is a recess after the prosecution rests, and another after the defense, before closing arguments and the jury's deliberation. During those recesses, unless they are sequestered, the jury members would return to their normal lives (including work), and report back to the court when summoned.