- Accidental Innuendo: The Red Moon of Meru features a doctor peddling the then-popular, now long discredited practice of measuring intelligence by the curves of a person's head. He walks up to a fashionable lady and offers to "feel her bumps".
- Crowning Moment of Awesome: Father Brown is very awesome, but one moment that stands out is in "The Queer Feet," when he calmly tells a roomful of rich and privileged aristocrats that the Gentleman Thief has not only escaped, sacrificed his loot, but repented, the reaction is disbelieving. Father Brown calmly answers, "Odd, isn't it, that a thief and a vagabond should repent, when so many who are rich and secure remain hard and frivolous, and without fruit for God or man?"
"Yes," he said; "it must be very hard work to be a gentleman; but, do you know, I have sometimes thought that it may be almost as laborious to be a waiter."
- From the same story, we have another CMOA when Father Brown delivers An Aesop:
Bankes: "Hang it all! After all, he was a convicted thief!"Father Brown: "Yes. And only a convicted thief has ever in this world heard that assurance: 'This night shalt thou be with Me in Paradise.'"
- In the story The Two Beards, everyone is convinced that an ex-thief in the neighborhood must be the culprit. Father Brown insists that the man is innocent, and when they persist he shuts them up with a single sentence:
- Fridge Logic: In the first story, when Flambeau asks for the package, he reveals that he actually swiped it some time ago in the next sentence. Why would he then stay with Father Brown and ask for it at all?
- Flambeau likes to get fancy with his crimes. For example, at one point he lists various types of people that can be profitably robbed or swindled, along with the appropriate setting for the crime in each case. He certainly seems to have enjoyed The Reveal in this particular instance...
- Moral Event Horizon: The staff of St. Bridgit's Moral Welfare, featured in "The Bride of Christ", cross it through their stealing babies from their mothers over any moral deformity, and they even had at least some of said mothers locked up in insane asylums, effectively scarring them for seven life terms. Worse, the murders of the week took place because of it—the murderer of the week happened to be one of their victims. It's telling that even Father Brown thought the whole operation was morally reprehensible, well beyond anything he had ever encountered as an amateur sleuth.
- Spoiled by the Format: The first mystery, "The Blue Cross", is told through the Sympathetic P.O.V. of the great French detective, Valentin. So when the unremarkable, seemingly bumbling priest (who isn't even named Father Brown untill near the end) solves the mystery, it would've been a huge shock to readers when it was first published in a magazine. But the most likely place for a modern reader to pick up this story isn't titled The Complete Detective Valentin Stories, so it ain't that much of a surprise.
- Strangled by the Red String: Flambeau is mentioned to have "casually and almost abruptly fallen in love with a Spanish Lady".
- Uncanny Valley: Evoked in several stories, e.g., "The Head of Caesar."
- Values Dissonance: Chesterton's racial and national attitudes were actually very moderate for the early twentieth century, but some will often strike a sour note for modern readers in the midst of his most enjoyable works, as for example in "The God of the Gongs." His religious views, on the other hand, were entirely conscious, and will strike the reader as either refreshingly forthright or offensively aggressive, according to taste.
- This article published at the Golden Age of Detective Fiction Forum The Sins of the Saint: Racism in GK Chesterton written by a Chesterton’s fan, analyzes 15 Father Brown’s tales that seem to contain this and absolves some of Unfortunate Implications… and others not. It also points that a lot of classic authors of Detective Literature (Agatha Christie, McDonald, Burton Stevenson) also had racist views, and he asks the reader to take in mind the purpose of the work (they were not racist propaganda).