I'm not exactly sure what happened, but i was on my fifth cog, out of six, had just started from an intersection when i nearly fell off the bike, my right foot slammed to the ground and i heard something tinkling on the asphalt behind me. Initially, only half the cog came off. I tested it, it rode and 8 miles later the rest of it fell off. Shifting to the faster gears was a little difficult if i was coming to a flat after a hill, but it got me home.