Is there such thing as too much fermentation
Continued fermentation of wine produces vinegar, which is not necessarily a bad thing. However, further fermentation may encourage growth of "vinegar eels" (harmless worms, actually, but not very appetizing).

Vinegar itself can be metabolized by microorganisms. According to the Compendium of the Microbiological Spoilage of Foods and Beverages, "several molds, yeast and bacteria are capable of spoiling vinegar... [such as the appropriately named] Lactobacillus acetotolerans."