Refermentation in the bottle
Unexpected fizz, cloudiness, and in the worst case bottles that leak or burst.
What causes it
Residual sugar plus surviving yeast, bottled without stabilisation.
Detection threshold
Obvious once it starts.
What you can do
Chill everything immediately and open carefully. Assume every bottle is affected, because they are.
How to prevent it
Ferment fully dry, or stabilise with sorbate plus sulphite. Sorbate alone without sulphite invites a geranium smell instead.
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.
Lactic spoilage
Sauerkraut, mousiness, or in the worst case a slick, ropy texture as the bacteria produce polysaccharide.