Is there a really common grammatical error that makes no sense to you?

Of course, there’s the usual mix up between “your” and “you’re,” which can be pretty understandable. But I’ve had so many people message me saying “what are you up too?” That is, they use “too” rather than “to.” It’s not that big of a deal, I know. But I’ve wondered how so many people have managed to learn that. I’m a soon to be English teacher, and I wouldn’t let my students make little mistakes like that. Sometimes I feel like it’s the little mistakes that annoy me the most, haha.