As Matthew Brady ends his arrival speech to a crowd of supporters on the street, the 'old time religion' song is sung by the throng, but the extras following behind the car, are either completely out of sync with the song or they are merely mouthing along with the singing because they don't know the words. Some of the extras are not singing at all.
The film is set in 1925. During Drummond's cross-examination of Brady, he uses the word "sex", not to mean "gender", but as a shorthand for "sexual intercourse". The first known example of this usage was not until 1929.
E.K. Hornbeck (analogous to H.L. Mencken) calls the town "the buckle of the Bible Belt". Although this was one of Mencken's signature phrases, he did not start using it until 1926, one year after the events depicted.
While questioning Matthew Harrison Brady on the stand, Henry Drummond refers to Charles Darwin's book as "The Origin of the Species". The book's title is actually "On the Origin of Species" (or, more popularly, "The Origin of Species").
During the voir dire phase of the trial concerning jury selection, Henry Drummond is forced to use his limited number of peremptory challenges to disallow prospective jurors who are obviously not interested in being impartial in any way to the point where one likens Prosecutor Matthew Brady to God. In that situation, Drummond should have called for such obviously biased prospective jurors to be struck for cause, a motion that can used an unlimited number of times with the permission of the court. If the court, which itself has obvious signs of partiality itself in the story, had rejected such a motion, Drummond could have resorted to using his peremptory challenges.
During the courtroom examination of Matthew Harrison Brady by Henry Drummond, Drummond shows Brady a 10 million-year-old rock, which he places on the judge's desk. Later in the scene, the rock is back in his hand, and when he dismisses Brady, he places it back on the desk without ever having picked it back up.
During the street carnival scene when Drummond arrives, watch Tracy's tie knot change from shot to shot. Kelly's makes a significant change during the monkey meeting scene. When the camera is on the stage with the monkey, Kelly's tie has a deep dimple below the knot that it did not have before, and then when the camera moves down into the crowd for the handshake with the monkey, the dimple disappears. Back to the flat tie. Tracy's knot continues to flatten and move side to side, all the way thru the approach by McHenry and as they finally walk up to the high school boys, his collar is even flipped.
When Drummond is inquiring about the story of Jonah, he flips to a book mark towards the end of the Bible and opens it. The Job story is in the Old Testament, but the section Drummond flips to would be towards the end of the New Testament.
At approximately 1:07, Spencer Tracy's characters refers to latitude and longitude and states "minutes, degrees, and seconds". Though technically correct, it is unusual for people that are familiar with such to list in that order; typically, logically/mathematically when listing such, degrees, minutes, and seconds are stated as "degrees, minutes, and seconds" because that is descending/hierarchical order.
When the Bradys walk toward the hotel porch after the revival meeting, the lights and buildings of the town are clearly seen behind them. But when Drummond leaves Brady on the porch a few minutes later, the scene behind them, facing the same direction, is completely blank beyond a few bushes.