Wow, that's hilarious. I've exact encounters like that too – where I was simply unable to describe something because I take it for granted in everyday speech. It goes both ways though. I was speaking to a younger Russian guy who couldn't really explain much of anything to me. He would resort to talking about learning things through movies and how the meaning behind it all isn't that important. I beg to differ, but that ship has sailed and won't be coming back...

Another problem with these is becoming too comfortable in your language and not actually trying that much to speak the language you're trying to learn due to self-consciousness... at least it was with me. I think you need to have a thorough understanding of the language first in order to really practice with someone else, or it will pretty much fail miserably. It doesn't have to perfect or you wouldn't need the practice, but you should be able to express yourself in more than one way for most thoughts and ideas relevant to the topic at hand.