Something similiar happened to me. After finally flipping my hub from singlespeed to fixed i was out riding around and when i back pedalled i felt the fixed gear slipping. Then it finally slipped right off leaving me pedalling to no effect. Later, when i trying to fix the problem back home, i completely stripped the thread on the fixed side of the hub. F#@ken sucks because I was really enjoying riding fixed until then. Now I have to go and get a new hub.