Volatile acidity
Vinegar, and at lower levels nail polish remover from the ethyl acetate that comes with it.
What causes it
Acetobacter converting alcohol to acetic acid in the presence of oxygen, or a stressed ferment.
Detection threshold
Around 0.6 to 0.9 g/L acetic acid for most tasters.
There is no fix
Not reversible at home. Small amounts can be blended out; anything obvious cannot.
How to prevent it
Keep vessels full, keep fruit flies out, and never leave a stuck ferment sitting warm.
Others in oxygen and microbes
Oxidation
Bruised apple, sherry, nuts. Whites turn deep gold, reds turn brown at the rim.
Brettanomyces
Barnyard, plaster, band-aid, smoked meat. Some people like a little of it; almost nobody likes a lot.
Film yeast
A white or grey film on the surface first, then a flat, thin, faintly solventy wine as the alcohol drops.
Lactic spoilage
Sauerkraut, mousiness, or in the worst case a slick, ropy texture as the bacteria produce polysaccharide.
Refermentation in the bottle
Unexpected fizz, cloudiness, and in the worst case bottles that leak or burst.