Film yeast
A white or grey film on the surface first, then a flat, thin, faintly solventy wine as the alcohol drops.
What causes it
Candida or Pichia growing on the surface of a wine left with air above it, feeding on the alcohol itself.
Detection threshold
Visible before it is smellable, which is the one mercy of it.
What you can do
Rack from underneath without disturbing the film, sulphite properly, and fill the new vessel to the neck. Caught in the first days the wine is usually fine.
How to prevent it
No headspace, ever. This is the fault that punishes a half-full carboy faster than any other.
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.
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.