Some typos ARE common, though, particularly ones involving switched letters or pressing a key close to the one you want, such as "teh" (accidental, not purposeful) or "yuo." Everyone knows how to spell words like those (I hope), so they're typos because they involve a slip of the fingers. Just because one accidently does that pretty often doesn't mean it's not a typo.
One typo I seem to make from time to time is using "you're" when I should use "your." I definitely know the grammatical rule. It's kind of ironic because most people have the opposite problem, always typing "your," but more than likely it's because they don't know the grammatical rule, as opposed to making a typo.