Lactic spoilage
Sauerkraut, mousiness, or in the worst case a slick, ropy texture as the bacteria produce polysaccharide.
What causes it
Lactic acid bacteria working on sugar or on glycerol rather than on malic acid, usually at high pH with low SO2.
Detection threshold
The ropy stage is unmistakable; mousiness is only detectable retronasally, after swallowing.
What you can do
Ropiness can sometimes be stirred out and the wine sulphited and filtered. Mousiness cannot be fixed at all.
How to prevent it
Finish fermentation dry, get pH under 3.6, and sulphite as soon as malolactic is complete.
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.
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.
Refermentation in the bottle
Unexpected fizz, cloudiness, and in the worst case bottles that leak or burst.