Commonest? Commonest? It made me cringe. I wouldn’t call it a mistake, per se, and perhaps Cecil meant to use it in jest; however, perhaps “most common” would’ve fit nicely.
Well, I wouldn’t call it a mistake per se, and I wouldn’t call it a mistake at all. Nothing personal, but I think that’s really more your problem than it is Cecil’s. [Dictionary.com cite] You’re probably one of those people who doesn’t like hearing funnest.