Warning: Spoilers are unmarked.
- Unintentional Period Piece:
- The novel is set in World War I and this informs the social conditions of the characters.
- The murder plan revolves around the use of both strychnine and bromides for medicinal purposes, neither of which are common past the mid-20th century.
- The plot of Alfred Inglethorp demands for him to be put on trial with weak and fabricated evidence, and to be acquitted. Even if real evidence surfaced later, he would then not be placed on a second trial due to the law on double jeopardy which "prevents an accused person from being tried again on the same (or similar) charges following a valid acquittal or conviction in the same jurisdiction." Thus Inglethorp would be getting away with murder. This is no longer the case. The Criminal Justice Act 2003 has permitted second trials in murder cases of England and Wales, if the Director of Public Prosecutions and the Court of Appeal agree to quash the original acquittal due to "new and compelling evidence".
- The Un-Twist: The novel has one of the most famous and well-implemented examples. Alfred Inglethorp is clearly set up as Emily Inglethorp's killer; he generally comes off as a shifty fellow and stands to gain the most from killing her, Evelyn vehemently insists he's the killer and all the evidence appears to point in his direction. But then more evidence is found suggesting he isn't the killer. But then, it turns out that Alfred is the killer and was actually trying to get himself suspected and then acquitted to exploit double-jeopardy laws, meaning that even if new evidence against him came to light he couldn't be tried for Emily's murder a second time. It also turns out that Evelyn is his lover and accomplice, with her insistence he's the killer being an act to take suspicion off herself and ensure Alfred's plan succeeds.
