Wine today is much more reliable than ever before. With the exception of a corked bottle, seriously flawed wine is relatively rare. There are, however, a number of problems you may encounter, which would warrant returning a bottle of wine to the place of purchase.
This is the most common wine fault, found in 2 to 5 per cent of all wines sold. It is caused by a mould found in some natural corks that can taint the wine, and has nothing to do with pieces of cork floating in your glass. Corked wine smells musty and lacks fruit flavours, but this may not become obvious until it has spent a few minutes in the glass. Plastic corks or screwcaps eliminate this problem.
Over-exposure to oxygen harms wine, eventually turning it into vinegar. A winemay become oxidized if its seal is insufficiently airtight, if left too long in bottle before opening, or if left too longonce opened.
All wines are bottled with a dose of sulphur, which acts as a preservative. However, if too much sulphur is added, the wine acquires an astringent, rubbery smell. In large quantities, it can be dangerous for asthmatics.
If you taste a wine with excess acidity, tannins, or oak, or with an absence of fruity flavours, it may simply be the result of poor winemaking.