Well, I converted my fixed gear Messenger into a freewheel... ...while I was riding.

When they say "check the lock ring before every ride" they mean it. Even with a brake it's scary as shxt when you go to backpedal and the pedals just spin.

I'm taking the wheel back to the LBS that installed my new cog and have them put it back on (with enough elbow grease that my legs won't take it off again.

Be safe out there.

Edit: I should mention that I was using a lock ring and the cog/lockring were both installed by the LBS.