Here's a report of a face-to-face transaction gone bad.

In this instance, a person selling headphones through Craigslist was contacted and a face-to-face trade (at a McDonalds) was arranged.  Except it didn't go according to the buyer's plan and the seller left with both the headphones and the bitcoins:

I just got robbed blind of bitcoins - in person. I'm feeling like I've lost trust in the usability of the currency and looking for advice.
 - http://www.reddit.com/r/Bitcoin/comments/1b89wm