A server at a steakhouse pours a splash of Cabernet into your glass, sets the bottle down with the label turned toward you, and waits. For plenty of diners these are the most nerve-racking few seconds of the evening, though the reason for the ritual is simple and practical. Somewhere between the vineyard and the table, a small share of bottles go wrong, and that first taste is the moment to catch one. If you have ever smelled a damp basement or a soggy cardboard box, you already have most of the equipment you need.

A wine flaw or fault is an undesirable aroma, flavor or structural condition caused by chemical, microbial, storage or production problems.

People in the trade use "flaw" and "fault" for any smell or taste the winemaker never intended, particularly one that crowds out what the wine is supposed to offer. These problems can start anywhere from the vineyard to a sunny shop window. Nearly all of them are harmless to your health. A flawed bottle simply tastes worse than it should, sometimes dramatically and sometimes so subtly that you only sense something is missing. The line between a flaw and a quirk is also blurrier than most textbooks admit, and experienced tasters argue about it constantly.

A broken or crumbling cork may indicate age or dryness, but it does not automatically mean the wine is corked. Cork taint refers primarily to compounds such as TCA affecting the wine itself.

Where Does a Wine Go Wrong?

It helps to sort flaws by where they begin. Some start in the vineyard. In 2020, wildfire smoke hung over Napa and Sonoma long enough that several estates chose not to make a vintage at all, and some wines that seemed clean at harvest showed an ashy, cold-fireplace character only after fermentation. Other flaws grow in the cellar. Brettanomyces, a yeast that thrives in barrels, brings smells of leather, barnyard, and adhesive bandages. Too much volatile acidity pushes a wine toward vinegar or nail polish remover. Reduction, which happens when a wine is kept too far from oxygen during winemaking, smells of struck matches or rotten eggs, and it is one of the few faults that often fades with air.

Those categories leak into one another, though. Cork taint usually comes from one bad stopper, yet the compound behind it can also settle into a winery's treated wood and other cellar materials. Château Ducru-Beaucaillou in Saint-Julien battled this kind of problem for a run of vintages from the mid-1980s into the 1990s, until a new cellar ended it. Natural wines made with little or no added sulfur are more prone to mousiness, an aftertaste reminiscent of a hamster cage that appears several seconds after you swallow, and some people cannot detect it at all. Brettanomyces has its defenders, too. Older vintages of Château de Beaucastel in Châteauneuf-du-Pape carried enough of it that many collectors came to treat the earthy, smoked-meat note as part of the house style.

Cardboard, Stewed Plums, and Burnt Sugar

Corked wine is the most common reason a bottle gets sent back, and it has nothing to do with crumbs of cork floating in the glass. The culprit is a compound called TCA, which forms when common molds meet certain chlorine-based chemicals, and the human nose can find it at levels measured in parts per trillion. A badly corked bottle smells of wet cardboard or newspaper left out in the rain. A lightly corked one is trickier, because its main symptom is absence. The fruit seems muted, the finish short, and the drinker usually decides the wine is just dull. Estimates of how many cork-sealed bottles are affected run from under one percent, by the cork industry's count, to several percent in professional tasting panels, partly because sensitivity to TCA varies so much between people.

Cork taint commonly creates musty aromas reminiscent of damp cardboard or a moldy cellar.

Cooked wine has been damaged by heat, whether in a delivery truck in July or a car trunk on the drive home. Fresh fruit turns stewed and jammy, closer to prunes or plum preserves, and the wine tastes flat. Maderized is the related term, borrowed from Madeira, where producers heat their wine on purpose, either in warmed tanks through a process called estufagem or in casks left under the warm roofs of the island's wine lodges. The nutty, burnt-sugar result is the whole point of Madeira. In a table wine, that same browned character means oxidation has set in, usually with heat's help. Cooked describes what happened to the bottle and maderized describes how it now tastes, so an overheated wine is often both, though an old white can oxidize in a perfectly cool cellar.

Mold on the exterior of bottles is common in cool, humid traditional cellars and does not necessarily indicate damaged wine.

Many Clues Sit on the Outside of the Bottle

You cannot smell TCA through glass, but heat, light, and age often leave visible evidence. A cork pushed slightly above the rim, a capsule that refuses to turn, or a sticky stain running down the neck suggests the wine got hot enough to expand. In older bottles, check the fill level, since wine that has dropped well into the shoulder has probably taken on air. Hold whites and rosés up to the light and be wary of a young Pinot Grigio that has gone gold. Under bright store lighting, wine in clear glass can pick up smells of cooked cabbage and wet wool within days, a flaw the French call goût de lumière, the taste of light. Most Champagne comes in dark glass for this reason, and Louis Roederer's Cristal, which keeps its traditional clear bottle, ships wrapped in tinted cellophane.

Where a bottle has lived matters as much as how it looks. A shop that feels warm, a pyramid of bottles in a sunny front window, or wine baking under hot display lights are all reasons to buy elsewhere or to reach for a bottle from the back of the shelf. At home, the cabinet above the refrigerator is one of the worst places to keep wine, and the floor of an interior closet is one of the better ones. In a restaurant, speak up if something seems off. A good sommelier would much rather open a second bottle than watch a guest quietly endure a corked one. Many retailers will also replace a corked bottle if you return it with the cork and most of the wine.

Proper wine evaluation focuses on aroma, flavor and structure rather than cosmetic imperfections on the bottle. Wine faults are easier to understand once appearance and condition are separated.

The Takeaway

The most practical thing a drinker gains from learning flaws is the ability to separate a bottle from a wine. A corked bottle says nothing about the vineyard, the vintage, or the people who made it, yet a lightly tainted one can quietly convince someone that a producer is mediocre. Winemakers lose customers this way and almost never find out. A drinker who recognizes that muted, faintly musty quality knows to give the wine another chance, and a server who can name it turns an awkward moment into a small act of good hospitality.

The list of flaws worth knowing also keeps shifting. Researchers in Australia linked wildfire smoke to tainted wine in 2003, and testing grapes for smoke has since become a familiar part of harvest across the fire-prone American West. Mousiness has been described for more than a century, yet it became a regular topic of conversation only as low-sulfur and natural winemaking grew. Screw caps, meanwhile, have made cork taint a rare complaint in New Zealand, where they became the standard closure in the early 2000s. A drinker twenty years from now may worry less about corks than about smoke. The arguments over Brettanomyces will almost certainly still be going.