Assuming it was a smooth-slip and not a clunk-slip, I'm gonna guess not chain, chainring, or cog related. Above comments rule most things out. So I'm gonna guess a slight rear tire slip due to rolling over something wet or slippery on the ground, especially if standing on the pedals, especially if it was just barely starting to rain, when the water forms an emulsion with road dust that is more slippery than after it really rains and rinses things clean.