UPDATE: Garmin contacted me today and wants to see the unit and mount. I'm happy to see them react proactively to the issue, and hope this is an isolated problem with an explanation. More information as and when I get it.
A few weeks back I wrote that the Garmin 500 had a new twist and lock mount, perhaps as a reaction to the problems that some users experienced with the slide-and-latch mechanics of its predecessors. Out riding yesterday, my Garmin 500 broke free from the mount and skittered behind me as I crossed a six lane road. I stopped quickly, laid the bike down and ran back to retrieve the unit (advantage mtb shoes) before cross traffic ran it over. Expecting it had just released, I was dismayed to find that pieces of the unit itself had cracked off in the mount (see photo showing one of the pieces). Examining the 500 closely revealed the plastic molded pins on the back of the Garmin unit itself had broken away. Nothing struck the Garmin during the ride, and the road I was riding on was perfectly smooth. I'm baffled.
I suppose I'll have to send the unit back to Garmin, which I'll do this week. This is the first time Garmin hardware has failed me, ironically with the new mounting mechanism. Perhaps the old one, wasn't so bad after all.