I'm partway through a Chapterhouse proofreading and copy-editing course and it has had an odd effect on me. (I should say that proofreading and copy-editing are things I have been doing for a long time, not things that are new to me; I just felt it would be a good idea to formalise my skills and obtain a certificate of competence to prove my ability to prospective employers.) I have long believed that my spelling and grammar skills are pretty good. Despite this, I routinely check words in the dictionary when completing exercises that are part of the course, or completing work for employers.
This has had a dual effect: on the one hand, it has shown me that I can spell and that my grammar is good, which is very reassuring. On the other, it has highlighted some words that I believed I knew how to spell and that I have apparently been mistaken about. For example, it turns out it's 'nerve-racking', not 'nerve-wracking', 'no one', not no-one', and 'doll's house', not dolls' house'.
These are only small things and yet they challenge assumptions and beliefs, which I welcome, since they also lead to learning and improvement. I expected to enjoy the course, and I do; as a bonus I am also, it seems, learning more than I expected to!