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: 
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.
(dangerous) overconfidence.

Although I agree with Terpsichore that the idiomatic expression isn't really too long or difficult to understand as-is.