Within the last half hour of the movie, the clock on the wall in the jury room can be seen indicating 6:15. Several minutes later, E.G. Marshall states that it is "a quarter after six". Several minutes after that, the wall clock is seen again, but still shows 6:15. Still later, when Lee J. Cobb leans over the table after he tears up the snapshot from his wallet, his watch can be seen indicating 5:10.
When Juror #8 wants to time how long it would take an injured man to walk down the hall, Juror #2 starts and stops the timing and announces it as "exactly 41 seconds". In reality, and considering that the scene does not cut away, the time is 30 seconds.
When the jurors take a washroom break, Juror #6 is fiddling with the fan's cord and eventually leaves it dangling a couple of feet from the fan. As Juror #8 comes out of the washroom, the cord is not dangling and is never seen again. Even when Juror #7 plugs it in after the light switch is turned on, there is no extra cord from the fan seen at all.
Juror 7 takes his coat off early on as do many of the rest of the characters. However, when the knife is summoned into the jury room and Juror 4 leans in and flicks it open, the very next shot is the reaction of Juror 8 sitting at the table. The partial body of Juror 7 is seen sitting next to Juror 8 with his coat on.
When juror #12 is sitting on the back of the chair smoking a cigarette, in the long shot the cuffs of his shirt are rolled back, but in the closeup the cuffs are fastened with cuff links, and then in the next long shot the cuffs are rolled back again.
Juror 4 is shown with a newspaper early in the film. When people are called for jury duty, they are explicitly told not to bring in local newspapers as reading material, as this could possibly give them bias in the case, should there be articles about it.
When Juror #4 stabs the knife into the table for the first time, about a dozen previous jab marks can be seen in the wood from previous takes.
When Juror #4 is asked to account for his evenings earlier in the week, he states that one on day he left his office. It is unlikely that anyone would be going to work in the middle of a murder trial, which the earlier conversation had noted had lasted for 6 days.
At the end, there is a shot from inside the cloakroom as Henry Fonda removes a coat from the hanger rod. As the camera tracks forward into the jury room, the hanger rod rises up out of the shot; the crew must have been lifting it out of the way to let the camera pass through.
Juror #9 reveals that be had noticed that a key witness that testified seeing the murder from afar when awakened in bed, had marks on her nose typically left by eye glasses, raising reasonable doubt the truthfulness of her testimony. In his subsequent line of questioning with the be-speckled Juror #4, Juror #9 then mentions he has 20/20 vision. Yet, upon subsequent close-ups, Juror #9 clearly has the same marks on his nose that the witness and Juror #4 had, indicating that he also wears glasses, invalidating his earlier claim of 20/20 vision.
During the rainstorm, the juror turns on the light by pushing the switch down. At the end of the movie, as they are exiting the jury room, the light switch is in the up position.
When Juror #8 is imitating the old man going to the door, as he is completing the first leg of his walk the shadow of the camera following him is clearly visible on the floor by his feet.
During close-ups, as many as seven studio lights at a time can be seen reflected in the eyes of various jurors.
(at around 1h 28 mins) The shadow of a camera can be seen on the back of Juror #3.
(at around 20 mins) As the camera pans from Juror #6 to Juror #7, the shadow of the boom mic is visible on the wall behind them.
When Juror 7 says, "This kid is 5 for O" he's speaking in baseball terms due to the fact that he's a baseball fan. However, the correct term would be, "This kid is O for 5." This may have been a deliberate choice by the director or actor to make Juror 7 appear ignorant and not knowing what he's talking about.
Juror #8 claims he's an architect and then eventually asks to see "a diagram of the apartment." No architect worth their salt would've said "diagram" in this context. He would've said "floor plan."
Juror 2 says he is waiting for the second hand on his watch to get to 60 before timing the walking demonstration. However, the second hand can be seen reaching the 10 seconds mark when he stamps his foot, indicating the start-off.