Faults

Diagnose it by what you can smell.

Sensory thresholds vary between people by an order of magnitude, which is why one taster is certain a wine is faulty and another cannot find it. Most of these cannot be reversed, and this page says so rather than selling you a cure.

Oxidation

Smells like: Bruised apple, sherry, nuts. Whites turn deep gold, reds turn brown at the rim.
Cause: 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.
Can it be fixed: Nothing reverses it. Blending away a lightly oxidised lot is the only real option.

Prevent it
Top up, exclude air, and dose sulphite to a molecular target rather than a free-SO2 habit.

Volatile acidity

Smells like: Vinegar, and at lower levels nail polish remover from the ethyl acetate that comes with it.
Cause: Acetobacter converting alcohol to acetic acid in the presence of oxygen, or a stressed ferment.
Detection threshold: Around 0.6 to 0.9 g/L acetic acid for most tasters.
Can it be fixed: Not reversible at home. Small amounts can be blended out.

Prevent it
Keep vessels full, keep fruit flies out, and never leave a stuck ferment sitting warm.

Brettanomyces

Smells like: Barnyard, plaster, band-aid, smoked meat. Some people like a little of it; almost nobody likes a lot.
Cause: 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.
Can it be fixed: Sterile filtration removes the organism. The aroma compounds already formed stay.

Prevent it
Molecular SO2 above 0.5 mg/L, clean barrels, and do not let a red sit warm with sugar left in it.

Hydrogen sulphide

Smells like: Rotten egg. If left, it develops into mercaptans that smell of onion, garlic or burnt rubber.
Cause: Stressed yeast, usually short of nitrogen, producing sulphur compounds during fermentation.
Detection threshold: Very low. Most people detect H2S at a few micrograms per litre.
Can it be fixed: Splash-rack early and it often blows off. Once it has become a mercaptan it is far harder, and copper additions are a last resort.

Prevent it
Adequate yeast nutrient, added in stages, and a fermentation temperature the strain can actually cope with.

Cork taint

Smells like: Wet cardboard, damp basement. At low levels it does not smell of anything: it just strips the fruit out of the wine.
Cause: TCA, formed when chlorine compounds meet mould, usually in cork but sometimes in the cellar itself.
Detection threshold: A few nanograms per litre; among the lowest sensory thresholds known.
Can it be fixed: None for the bottle. If several bottles are affected, suspect the cellar rather than the cork.

Prevent it
Never use chlorine-based sanitiser anywhere near a winery.

Refermentation in the bottle

Smells like: Unexpected fizz, cloudiness, and in the worst case bottles that leak or burst.
Cause: Residual sugar plus surviving yeast, bottled without stabilisation.
Detection threshold: Obvious once it starts.
Can it be fixed: Chill everything immediately and open carefully. Assume every bottle is affected.

Prevent it
Ferment fully dry, or stabilise with sorbate plus sulphite. Sorbate alone without sulphite invites a geranium smell instead.

Most of these trace back to sulphite management. Check yours against your pH.