99% of the time when someone asks this question on BF, their lockring was loose. This allows the cog to spin backwards when you slow down, then when you start up again it retightens, and you feel your cranks slip a quarter of a turn.

If you have a brake, it is perfectly safe to ride, just avoid any backpedalling until you can have the lockring and cog properly tightened. If you are brakeless, then do not ride it. You could strip your hub or much, much worse.