This has happened to me a couple times, but it was only this most recent time that I actually remembered the word. I was at a teppanyaki restaurant yesterday, and my chef was a Japanese man in his mid twenties. It was fun practicing my Japanese with him, but the conversation started out a little rough. He asked me where I studied Japanese, but did so using the word まなぶ (manabu) which I was unfamiliar with (and whose subtleties I'm still unsure of). According to my handy online dictionaries, the meaning is to study (in depth); to learn; to take lessons in. I've been taught benkyousuru (to study), and narau (to learn) both in class and through reading textbooks. I don't ever remember running across this word before in those contexts.
I'm not positive, but from the context it sounded like the verb implied where did I first learn Japanese. It might just be the case though that I was focusing so much on an unfamiliar verb, that I didn't hear some of the rest of the sentence. When I replied that I was studying with a private tutor, it seemed to be an incorrect response :) I guess I'll ask my teacher this week about manabu. I can't believe that in 2.5 years I've never learned that word before.