It happened to me in 6th on a 7 speed cassette. It started during a left hand turn across 2 lanes of traffic by making some "something's breaking" sounds but got me through the turn. Next time I shifted to that gear there was more noise followed by less resistance pedaling, and when I stopped to see what was wrong half of the gear was left. It was on a Performance cassette, and they gave me a break on a Shimano cassette that's been fine for a few years.