Brettanomyces
Barnyard, plaster, band-aid, smoked meat. Some people like a little of it; almost nobody likes a lot.
What causes it
A spoilage yeast in reds, thriving at high pH and low free SO2, often living in old oak.
Detection threshold
Widely variable. Some tasters cannot detect it at all.
There is no fix
Sterile filtration removes the organism. The aroma compounds already formed stay exactly where they are.
How to prevent it
Molecular SO2 above 0.5 mg/L, clean barrels, and do not let a red sit warm with sugar left in it.
Others in oxygen and microbes
Oxidation
Bruised apple, sherry, nuts. Whites turn deep gold, reds turn brown at the rim.
Volatile acidity
Vinegar, and at lower levels nail polish remover from the ethyl acetate that comes with it.
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.