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 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.
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.
Juror 7 takes his coat off early in the film as does 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. You can now see the partial body of Juror 7 sitting next to Juror 8 with his coat on.
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.
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.
Jurors are expressly instructed by the judge in every jury trial to only consider the evidence that was actually admitted into evidence. Thus, it was misconduct for Henry Fonda's character to go out and purchase a knife on his own, and it was misconduct by all jurors to discuss that knife during deliberations. Further, they had a duty to inform the judge of the misconduct that had occurred in their deliberations. The acts of jury misconduct were egregious and absolutely were grounds for mistrial.
Juror #8 (link=nm0000020]) bought a knife and shows it to the jury in order to refute the idea that the boy's knife is one of a kind. As pointed out in the discussion revolving around the knife, it is illegal to buy/sell/own a switchblade knife. After the jurors have left the room at the end, the camera pans across the jury room table and the knife #8 purchased is still laying on the table. Whoever cleans that jury room and finds that knife will certainly report it to the bailiff or an officer, thus leaving #8 open to possible arrest for possessing the knife should they investigate and learn where the knife came from.
The film is implied to take place on the East Coast of the United States, based upon the baseball teams that the jurors mention. Also, one of the jurors (Juror 12, played by Robert Webber) asks Juror 8 (played by Henry Fonda) whether they can see Manhattan's Woolworth Building from the jury room window, which Juror 8 affirms. The opening/exterior shot of the court house is that of the New York Supreme Court in Manhattan at 60 Centre Street.
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.
At the end of the film 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.
After the men begin taking their seats at the beginning of the movie, the camera pans over to Juror #8 staring out the window. The shift in perspective as the camera moves towards him makes the building props in the background very obvious.