I've been watching a lot of Scrabble videos on YouTube. I don't know why as I've never played the game ... however, there's a phenomenon where the best players in the world are challenging really obvious words because of the quirks of the language.
In the video below you can see an example of where someone challenged "bus" because "bu" isn't one of the many two-letter words they've memorized and so pluralizing it is also invalid.