i've had a similar problem. i went on a ride to the beach and heard squeeling noises coming from my bottom bracket. then the crank just slipped forward when i applied pressure hard from a stop. it slipped almost a half turn. i dont know what the problem is exactly, but my lockring and cog should be properly tightened. i havent done anything to the cog since i originally got it, but i do check my lockring to make sure its tight from time to time. everything seems to be properly tightened so what might the problem be?

i looked at the markings on the bottom bracket and they dont change position at all, so i'm left to think it must be something with the spindle? the cranks are not stripped btw, they are also in good shape. any ideas?