Danish Birgitte Poulsen tells, that Virginia Welton's married name was Østergaard, but on the sticker she places on the board, it is spelled Astergaard. The correct transcription from Østergaard into English would be Oestergaard.
In one scene a gloved hand sets a butterfly on a flower. But in reality one touch to a butterfly's wings can cripple it by removing the fragile pixie dust-like scales and render it unable to fly.
The 'Circulus Entrance Exam' has the spelling error 'Instuctions' above (1) on the paper instead of 'Instructions'.
A woman as intelligent and precise as the Danish detective would not commit the grammatical error of saying "me and my father". Even functioning in one of the several secondary languages she knows, that type of mistake is beneath her.
Of the choices in the first quiz question Grady is given, the best answer for Roadman is D, a rock band assistant, which is what he chose but the quiz counted it as incorrect, although it may have been designed to count all answers incorrect. For the next question, he did choose incorrectly, Chocolate is by The 1975, not the Arctic Monkeys.