We've gotten some serious snow in Montreal lately and it was snowing today so I decided to take the fixie out for a little fun. While I was riding through a particularly bumpy patch of snow, I tried a little whip skid, just for kicks and giggles. Lo and behold, my chain fell off. Luckily I wasn't going too fast and I don't ride brakeless. I was able to hook the chain on the top of the chain ring and peddle it back on. The tension was certainly adequate - there was no discernible play in the chain. The cog and lockring are secure. After I got home I took off the chain and discovered that the chainring was the slightest bit loose. With the chain on I couldn't detect any play at all. What's the deal? Cog doesn't look that worn out?