Question: Every sector has them: the employee who has had very little formal training about a certain program, device or concept, but has done research into it himself and figured out just enough to have a negative impact on the normal operations.

I'm talking about something like a manager who knows just enough about Outlook to make a software extension to regularly delete some unneeded contacts, but not enough to realize that this extension will remove those contacts from the contact managing server as well. Or a soccer mom who knows that they can refill their oil under the hood, but can't tell their oil tank from their wiper fluid canister.

I'm looking for a way to express this in as little words as possible, preferably a single word, but 2 or 3 words will also be fine. I'm specifically looking for an adjective, something you can say about a person like "He's X" or "That X man".

Answer: 
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".
People used to say "sophomoric", as in somebody who's had just enough education to start making foolish selections from their new store of knowledge. "Sophomore" (do they still call second-year college students sophomores?) comes from the Greek roots for "wise" and "foolish".

I haven't heard the usage in years though, so you might end up just puzzling your audience.