Or does the restaurant eat the loss? What usually happens?
No reputable restaurant would charge you for an undrinkable bottle of wine. They would either eat the cost or push it back to their supplier. But bad bottles of wine are just one of the prices of doing business.
Someday, when wine switches to screw tops (which have much less chance of spoilage) this will be less of an issue.
What Telemark said. If you just don’t like the bottle, you are out of luck if you are honest. A restaurant will probably bring you a new one just to keep your business if it isn’t too pricey, but otherwise, if it is drinkable, you bought it. That’s the chance you take with older wines. Estimates run 3-5% tainted or corked. Price of doing business. Bring on the screw-caps and screw the cork-snobs! 
The OP said that it was “undrinkable.” A restaurant will always pour you a little bit to try for your approval, right? If it’s gone bad, they will not charge you.
If you buy it at a wine auction, you are SOL.
At a restaurant, they’ll bring you a new bottle and eat the loss (actually, I believe distributors give a rebate, so it’s not really much of a loss, if at all). From a retailer or auction or whatever, you’re probably SOL.
There are people who get a charge out of sending expensive bottles of wine back–I beleive they’re cousins of the guys who jump at the opportunity to shout “CITE?” on message boards–but according to The Playboy Advisor, there are only two situations where that’s acceptable: The wine has turned to vinegar, or there are pieces of cork floating in it.
Restaurant eats the cost.
:rolleyes:
Then The Playboy Advisor is wrong.
Off the top of my head I can think of at least 4 or 5 reasons that would be acceptable for returning a bottle of wine. And cork floating in the glass isn’t one of them.
Corked - I see this term being misused fairly often. It does not refer to wine that has been exposed to air (oxidized.) It means the natural cork (or a barrel or possibly some other wooden equipment in a winery) used contained TCA, or 2-4-6 trichloroanisole, a compound that usually presents itself after the use of chlorine, which contaminated the wine. A corked wine will smell like a dirty old newspaper, or a musty basement, or a moldy old piece of cardboard, something like that. Different people have different levels of perceiving taint in wine. I know I can detect it much more easily than my husband can, which is why I’m always the ‘taster’ when we order a bottle while dining out.
Oxidized - Air has crept into the bottle somehow. Usually indicates the bottle wasn’t stored properly, on its side to keep the cork moist. It may be an off color, and will smell like Sherry or over ripe fruit, something like that.
Maderized - It was stored for a prolonged period at too high a temp. It’s ‘cooked’ or ‘baked.’ It will smell and taste like Madeira, sweet, cloying, maybe nutty. Obviously some of those characteristics are desirable in certain varietals, particularly dessert wines, but a dry usually shouldn’t have them.
Volatile - The wine was spoiled by a bacteria and is essentially turning to vinegar. It will smell, well, like vinegar.
Brett infected - The wine may be spoiled by a yeast called Brettanomyces. In some, usually European wines, Brett is desired if it’s in low levels. Most new world wines shouldn’t have characteristics of brett. If the wine smells like a barn or an old horse blanket, send it back.
Refermented - Some yeasts may referment in the bottle. If the wine feels bubbly, tingly, almost effervescent on your tongue, it may have refermented in the bottle. Again, desirable in some wines, like sparklings and Champagne, but you don’t want a bubbly-feeling still wine.
The wine is misrepresented by the sommelier - If you ask for something dry and the sommelier recommends one then brings you something sweet, I’d say you have a right to send it back. Or he may have just brought you the wrong wine. I have sent one bottle back, before it was opened; when the server presented the bottle, it was not what I had ordered (wrong vintage, which in most cases I wouldn’t care about, but I happened to be very familiar with this particular wine’s vintage history and wanted a certain one.)
For cork floating in a glass, I wouldn’t send an entire bottle back. I’d either fish it out, or ask the server to strain the wine.
Like Telemark said, any reputable restaurant will take the bottle back. In all cases I would ask the sommelier to taste it to confirm your opinion however. And as silenus said, if you simply don’t like the wine and it is not flawed in any way, then you are just SOL. A restaurant may take it back, but they won’t be too thrilled about it.
The restaurant may get a credit from the distributor, who may or may not get a credit from the winery. If you purchase a flawed wine and bring it home, you can try to call the winery to see if they are willing or able to replace it or give you some kind of credit. Some will.
FYI - There is now a machine that can scan a bottle of wine to detect if it is flawed. As of right now there is only one in a restaurant somewhere in New Jersey, but I’ve read that auction houses like Zachy’s and Christie’s are interested in the technology.