"Screenplay" is misspelled as "Screemplay" in the opening credits.
When a newspaper close-up is shown, with a report of a murder on a train on the front page, the French word for wife (Epouse) is misspelled.
Dry blood on second victim seconds after murder.
When John strikes at second victim, he clearly misses her on purpose and would maybe hit her on head rather than neck.
Though John is earlier shown scattering his wife's ashes to the rain, her ghost ends up in the last scene with the bag of her ashes intact, sifting them through her fingers.