Your kombucha should have a neutral odor at first, and then gradually take on a more vinegary smell. If you begin to notice a rotten or unpleasant odor, toss the liquid, and carefully examine the SCOBY for any signs of mold. If the SCOBY has no mold, simply start again with fresh filtered water and starter tea. If there is mold on the SCOBY, toss everything and start again with all new ingredients.