The Thirty-Nine Steps/YMMV

Contrived Coincidence: The series is known for its many improbable coincidences.
- Out of the entire housing stock of Scotland, Hannay just happens to enter the house being rented by the spy ringleaders?
- In Mr Standfast the remote and inaccessible Scottish cave Hannay is staking out is visited on that very evening by a possible antagonist from earlier in the book. He turns out to be a complete innocent who likes mountain climbing and just happens to be in the area.
Gentleman Adventurer: The Thirty-Nine Steps takes place after Hannay has retired from a busy and dangerous life as a mining engineer in Africa; he does have the leisure to pick up Scudder's adventure, but rather jumps at the opportunity because the idleness is driving him crazy. Then it's averted in the sequels, with Hannay becoming a hard-working Army officer.
Magnificent Bastard: Most of the villains.
Nightmare Fuel: Mary's description of what she's going to do to the bad guy in The Three Hostages makes the good guys quake in their shoes.
Fair for Its Day / Values Dissonance: Buchan's writing can tend towards the jingoistic when read with modern eyes, but it's nowhere near as bad as some other authors writing at the time; in fact, when compared to works such as Bulldog Drummond, Hannay and Buchan are downright progressive. Greenmantle in particular shows detailed knowledge of and respect for Islam.