Often times the waiter will inadvertantly or advertantly bring a wine of a different year(lesser vintage) than the one you ordered. You should read the label BEFORE he removes the cork to make certain he brought what you ordered.
Or sometimes a different wine altogether. I actually had one waiter pour the "test" sip with his hand over the label, in an obvious attempt to hide it - he came to the table with the bottle already opened. Needless to say I rejected it without tasting (it wasn't what I ordered - he said they were out of it). I'll spare you the remainder of the story. Aside from this, I've probably sent back 10-12 bottles over twenty years, and we eat out quite a bit. When in doubt, ask the server what s/he thinks. In nicer places, there's usually no question, they will agree with you. One bizzare circumstance forced me to send back a bottle of Champagne - it was flat. It had the characterstic "pfuff" when opened, but then nothing. Another reason to sample before allowing the bottle to be served is to judge the temperature of the wine. In the US, the whites tend to be too cold, the reds are often too warm. Sampling gives you the chance to say "this white is too cold, leave it on the table, don't put it in an ice bucket..." before the server leaves. Inspecting the cork is important. First, look at the condition. If it is dried out, and/or the server had problems extracting it, pay closer attention to the sip of wine. This cork condition could indicate incorrect storage conditions - or simply a flawed cork. Second, the cork can help confirm the wine is what you ordered. Almost all wineries put their name on the cork, and some also put the vintage on the cork as well (many Bordeux, some Burgundy and Italian reds. I've not seen this as often on American wines). If the year on the cork doesn't match the label, well, you've got a discussion and decision to make. There might be one other area for discussion with the server. I agree it is easy to detect a spoiled or flawed wine; however, some fine wines have what I would call a "barnyard" aroma. I've seen this most often with older red Burgundy. After 20 minutes or so of the wine being in the glass/decanter this will usually dissipate. I cannot describe this any better, so if presented with such a bottle it might be good to get agreement that if that nose doesn't dissipate (and assuming you don't like it), the bottle would be replaced. Now let's all cheer for a better closure: the screw cap.