I once flipped over my bars in a sprint when clipping out of SPD's--breaking collarbone--2 months later I was in a bike shop telling my story when mechanic lifted his shirt--broken collarbone a month before mine from---clipping out of SPD's. I'm new to fixed and having a little trouble clipping into my road pedal while it is moving ("damn--stay still") and thought about putting on my SPD's--that thought lasted about 5 minutes.