Past its best-by date? Impossible!
One question we often hear is: how long does vinegar keep? The simple answer:
In a closed bottle, vinegar can be stored indefinitely, preserved by its own acidity. Once opened, fruit vinegars can be stored at room temperature for many weeks. If it is going to take longer than that to use it up, however, fruit vinegar should be stored in a cool place.
Over the course of time a bit of sediment may develop at the bottom of the bottle. This is mother of vinegar and a sign of naturalness. It has no effect on either the quality or shelf life of the vinegar. If it bothers you, strain the vinegar and continue to use it as before.