Oxidation
Bruised apple, sherry, nuts. Whites turn deep gold, reds turn brown at the rim.
What causes it
Oxygen exposure, usually through headspace, a leaking bung or too little free SO2 for the wine's pH.
Detection threshold
Detectable long before it is obvious; colour usually moves first.
There is no fix
Nothing reverses it. Blending a lightly oxidised lot into a sound one is the only real option, and only if the fault is slight.
How to prevent it
Top up, exclude air, and dose sulphite to a molecular target rather than a free-SO2 habit.
Others in oxygen and microbes
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.
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.