Wine faults are specific chemical problems rather than matters of taste.
Cork taint
A compound producing a damp cardboard aroma.
Which affected a meaningful proportion of bottles historically.
Oxidation
Air exposure producing nutty, flat characteristics.
Which is a fault in most wines and a style in a few.
Volatile acidity
Vinegar and nail polish notes.
Which is acceptable in trace amounts and unpleasant beyond.
Reduction
Sulphur compounds from lack of oxygen.
Which frequently blows off with air.
How common these actually are
Cork taint affected a meaningful percentage of bottles before the industry addressed it through improved cork treatment and alternative closures.
Which is why screwcaps became widespread, and the quality argument against them has largely been settled.
Most faults a drinker encounters now are storage-related rather than production-related: heat damage and oxidation from poor handling.
Storage
Temperature stability mattering more than absolute temperature.
Which is why fluctuating conditions damage wine.
Light damage
Particularly affecting sparkling and white wines.
Which is why bottles are coloured.
Returning a faulty bottle
Retailers and restaurants generally accepting genuine faults.
Fault or style
Some characteristics deliberate in particular wines.
How to tell a fault from a preference
A fault has an identifiable cause and a recognisable character: damp cardboard, vinegar, wet wool, or cooked fruit from heat damage.
Which is different from simply not enjoying a style.
Restaurants and retailers generally replace genuinely faulty bottles without argument, and a description of what is wrong makes that conversation straightforward.
Closures
Cork, screwcap and synthetic with different characteristics.
Which is a production decision rather than a quality signal.
Sediment and crystals
Harmless deposits frequently mistaken for faults.
Which indicate minimal filtration rather than a problem.
Decanting
Aeration helping some wines and not others.
Serving temperature
A more common cause of disappointment than any fault.
What most people encounter
Not production faults but wine that was stored badly, served at the wrong temperature, or opened too long ago.
Which are all fixable and none is the producer's responsibility.
A bottle left in a warm car or beside a radiator can be damaged within hours, and the resulting flat, cooked character is frequently blamed on the wine.
Once opened
Oxidation beginning immediately.
Which stoppers and preservation systems slow rather than prevent.
Serving temperatures
Reds frequently served too warm and whites too cold.
Which affects perception substantially.
Glassware
Shape affecting aroma delivery measurably.
When in doubt
Asking staff, who deal with genuine faults regularly.
Why the production side is worth knowing
Drinks are among the most heavily marketed products there are, and a great deal of what is said about them is designed to justify a price rather than to explain a process. Heritage, craft, tradition and small batch are all words doing commercial work.
The processes underneath are genuinely interesting and largely documented. Fermentation, distillation, ageing and blending are technical disciplines with real constraints, and understanding them makes it considerably easier to tell the difference between a product that reflects a genuine method and one that reflects a design brief.
It also makes the money easier to place. Knowing that duty is a large share of a cheap bottle, that ageing costs a producer volume every year, and that appellation rules constrain method rather than guarantee quality explains most of what price actually reflects.
A note on drinking
Nothing here is intended to encourage drinking, and the health evidence on alcohol has moved in one direction over recent years. Guidance in several countries has been revised downwards, and no health body now describes alcohol as beneficial.
Anyone concerned about their own drinking, or someone else’s, can get free and confidential help from national health services and from support organisations in almost every country. That help is available regardless of how much someone drinks and does not require anyone to have reached a crisis first.
A general note
Production rules, labelling requirements, duty structures, licensing and health guidance all differ substantially between countries and change. National regulators, tax authorities and health services publish the applicable rules and are the authoritative source.
A closing thought on the subject
Drinks are one of the oldest manufactured products there are, and almost every technique in current use was arrived at empirically long before anyone could explain why it worked. The science came afterwards and mostly confirmed what producers had already established by trial and error over centuries.
That history is worth holding onto when a product is marketed as an innovation. Most of what is genuinely new in this industry is in measurement, consistency and scale rather than in method, and there is no shame in that: reliably making the same good thing every time is a harder achievement than it sounds.
Where to read further
Trade bodies, production institutes and regulators publish technical material on all of this, most of it written for producers rather than for consumers and consequently much clearer about what actually happens.
Brewing and distilling institutes, oenology departments and national food standards agencies are the obvious starting points, and almost all of their introductory material is freely accessible.