The box art for Fire Emblem: Three Houses features the character Claude upside-down, seemingly as a stylistic choice meant to form a triangle with fellow House leaders Dimitri and Edelgard while crossing their weapons. However, he is mistakenly shifted left-of-center to the point where his bow does not appear to line up with the weapons. This odd placement led to a recurring in-joke among fans where Claude would frequently be seen upside-down in memes and fan works. This joke was acknowledged by Nintendo and staff involved with the Fire Emblem series on three known occasions. Ippei Soeda, a character artist for Fire Emblem Heroes, posted his own illustrations of an upside-down Claude days after the box art's reveal in March 2019. Later in July, voice actor Joe Zieja hung upside-down by his feet from his back porch as he announced he would provide the English voice for Claude in the game. Nintendo of America's Twitter account later acknowledged the joke in 2020 by tweeting "Happy Birthday Claude!" upside-down and backwards.

According to director Toshiyuki Kusakihara, he wrote out an extensive history for the continent of Fódlan from the beginning and containing "probably a good ten-thousand years' worth of material" just to use as reference materials for development. However, he has stated that he would not release it, and expected more inquisitive players to analyze the story and fill in the holes themselves as the developers gradually forget the material over time.