so I took it in to my LBS today and they said my lockring was loose. the mechanic tightened it up and I sprinted a bit and the catastrophic-sounding problem is gone, but if this happened twice within a month, shouldn't I be a little worried about my lockring/cog situation? it was suggested that I come back to the shop in a week to see if it happens again, which then they could try and fix it for good with new parts under warranty, but is there anything to do on my part as a preventative measure, or am I totally at the mercy of my LBS?