Most of the time I'm writing about what's good about wine and trying to pick out wines that have a particular characteristic to delineate a point. But the fact is that wines aren't always good and it's worth having a look at why that should be. Now I'm not talking about wines whose taste you don't like for whatever reason - I mean wines that are flawed in some way, wines that have something wrong with them.

The most common fault, and possibly the most easily recognised, is corked wine. This isn't actually the wine-maker's fault; it's caused by the interaction of the bottled wine with an infected cork, which gives the wine a woody taste and a very unpleasant smell. Like all faults with organic products it comes as a spectrum, from the barely perceptible to the nose-wrinkling awful. If you ever wondered why sommeliers sniff the cork after extracting it from a bottle, that's the reason, they're checking for the 'corked' smell.

When you're somewhere where wine is made, you have the opportunity to taste wine that isn't filtered and isn't stabilised. Fresh, raw wine like this can easily become vinegar. Again, like the 'corked' syndrome, it's a spectrum. Sometimes you can be into your second or third glass before the unmistakable taste of vinegar makes itself felt. In Ireland, unless you make your own wine, you're unlikely to find this phenomenon. Commercial wines are made in carefully maintained environments up to the moment of bottling, making vinegar next to impossible.

Oxygen and wine don't go well together. If a bottle is stored standing upright the cork can shrink as it dries out, allowing the ingress of air and consequently oxygen. A bottle that's heavily ullaged - that's to say whose level in an upright position is well down the shoulder - should be regarded with suspicion. If that much air has got into the bottle the wine will almost certainly be undrinkable. In white wines oxidisation makes it change colour from a pale straw to a deep honey and imparts a taste reminiscent of Madeira, hence the term 'maderisation'.

There are also percieved faults; faults that to the wine-maker are merely symtoms of a well-made wine, but that to the consumer are defects. Sediment in red wines comes into this category. Increasingly wines are being filtered in the wineries because consumers are complaining to retailers about sediment in their red wines, and returned bottles eventually are paid for by the producers. To keep returns to a minimum reds are now almost universally filtered, a process that makes little difference to wines that are drunk young, but can have a significant effect on wines as they age - filtered wines losing more of their character than similar unfiltered ones.

Wine of the Week.

Domaine de L'Arjolle Paradoxe 1999

Over the last ten years I've often enjoyed the Cuvee L'Arjolle, which is now represented in Ireland by Mitchells. It's a fine wine that retails at €11.80, but if you want something very special, the new 'Paradoxe' is exceptional, although more expensive.

Available from Mitchells in Kildare Street and Sandycove, RRP €26.50