Has anyone else came across their gear shift lever detaching from their bike?

This fucked my mind in all sorts of ways this evening.

Coming back on the motorway come down the off-ramp, braking, clutch-in go to tap down a gear or three and I nearly lose my foot under the wheels because there was nothing to hit. The pivot of the gear shift pedal had somehow managed to unscrew itself out of it's mount and the lever was hanging off the bottom of my bike by the connector to the gearbox.

Never in my five, fine years of riding have I ever came across this stupidity before. Got it back on and got back home about and hour and a half later than I would have liked but still.

Anyone got any suggestions on how this happened? Could it just have been shaken out by engine vibrations, or just loose? Even better does anyone have a suggestion on how to avoid it happening again!

Bike is about a year and half old with about 2150 miles on the clock. It's a weird one.