During the trial, Hornbeck hands Cates a drawing of a stick figure swinging on a gallows. The defendant wrinkles the paper in his hands immediately. The scene changes and the defendant is holding a cup and not the piece of paper.
E.K. Hornbeck calls the town "the buckle of the Bible Belt". Although Hornbeck represents H.L. Mencken who popularized the term "Bible Belt" in 1926 and might have coined it earlier, the movie wrongly implies that the term was already well known in 1925.
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.
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 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.
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.