I reckon 'you' is my most common typo.
I often type “you“ when I mean to type “you're“ or “your”.
That's funny too, because I rarely mix them up when I spell them properly ( which is more than I can say for a lot of people ).
I hate typos so much.
On a related note, I think some valuable education software would be a tool that could look over a collection of an individual's work ( such as their notes, or web log, etc. ) to find all the common typographical errors and spelling mistakes. That way they could find problem areas and fix them.