Learn English – Term for talking about a subject or idea you did not want to talk about

An applicable neologism is humblebrag, a boast couched in self-deprecation. Henry Alford describes it as the false modesty of a plea intended to be met with both awe and sympathy. Humblebrag is attributed to Harris Wittels, a writer for the NBC sitcom Parks and Recreation who Tweets celebrity examples at that handle and has collected them into a book.

A less popular term from a different U.S. sitcom is backdoor brag, from a 2008 episode of 30 Rock. While not as buzzworthy a term, the example used in the episode matches one of yours:

It’s sneaking something wonderful about yourself into everyday conversation… Like when I tell people it’s hard for me to watch American Idol, because I have perfect pitch.”

Update: J.P. Freire in the American Spectator (a politically conservative U.S. magazine) proposes whinebrag, photobrag, busybrag, sorrybrag, and emobrag to capture additional flavors of show-offy self-deprecation. Who knows if any of them will gain currency; he notes the overlap of whinebrag with first-world problems a.k.a. white whines, and the Harvard Business Review lamented the busybrag months ago.

You'll probably get quite a few suggestions, but unless they're really good I'd stick with your own wording, "knowing just enough to be dangerous". It's good as it stands, and isn't too long either. But I'll just suggest replacing "dangerous", because as commonly used it implies danger in a wider area than that of mere technical competence. Someone might be described as "dangerous" if they are a threat to the politics of the office, a back-stabber, a gossip, a schemer. To restrict it to the area you're talking about, I'd say "knowing just enough to be a hazard", or "just enough to be a menace".