Wines have corks. So why is "corked" a bad thing?
When sommeliers say a wine is “corked”, they’re not referring to the fact that the cork is indeed in the bottle, but rather a fault with the wine, which originated in the cork, called TCA.
TCA, or trichloroanisole if you want to get technical, is a chemical compound resulting from little bits of fungus reacting with the wood in the cork. It's hard to detect in the cork before bottling, and even a tiny, teensy bit of TCA can completely ruin a bottle of wine.
So what is the problem? You’ll know it when you smell it. Corked wines smell like wet newspaper, wet dog, musty basement- not easily confused with typical wine aromas. The cork itself might look dark and musty... though it could also look totally normal. If you suspect a wine is corked but aren’t sure, wait five minutes and smell it again. Once exposed to oxygen, the odor intensifies quickly.
Aside from oxidation (the exposure of wine to air before the bottle is opened), cork taint is the most common fault in wines, with somewhere between 2-7% of wines bottled with corks affected. The good news is, most good wine stores will replace a corked bottle for free, and good restaurants won’t even serve a corked bottle in the first place. The other good news is, corked wine, though smelly, is completely harmless. So if you don’t mind drinking wet dog wine, or you just don’t feel like going back to the store, have at it. Waste not, want not. Right?