[jreeve]

IMO, when you are in a situation where you have to teach, ultimately you realize that you both know your subject material well enough to help out people who are new to the material even if you might get minor details wrong. And in the event that you get something important wrong (it's happened to me, FWIW), people know that you are human-- as long as you're willing to own up and figure out where you are wrong (and actually do have a bit of mastery over the larger material) most students are happy to be learning along with you. The only way to overcome is to actually make mistakes and practice dealing with them. Fortunately, the more you teach, the more you will find opportunities to deal with your misunderstandings of the world.

[personlurking]

Language-wise, though, while I might know a certain language up to a certain level of fluency, I won't offer to teach that level to someone else. If I'm fluent (or intermediate), I'll have no problem teaching up to intermediate (or beginner) level.

Teaching something to show you are retaining it isn't the best way to go, though teaching something to show you have retained it is better.