Aged sherry and Rioja wine vinegars.
For as long as wine has been made in Spain, vinegar has been a by-product. In a sherry Bodega, vinegar was seen as a failure, especially if too many of the butts turned. However, from about the 1950s things started to change. Sherry vinegar is now even protected and is one of only three vinegars in the word with Denomination of Origin.

With advances in science sherry vinegar is no longer a mysterious and spontaneous occurrence and now the best vinegars are deliberately made, often by topping up butts already containing vinegar with sherry. The alcohol quietly converts into acid, which can take months, even years, in the cool bodegas.