You're in a restaurant, have looked through the wine list, and you've told your server which bottle of wine you want.
Your server returns a few minutes later, doesn't say a word, and nonchalantly prepares to uncork your bottle of wine. Except it isn't your bottle of wine — your bottle was a completely different vintage.
The difference of a year can be extremely significant, especially for red wines. (A 2006 Grgich Hills Cab-Sav is not going to drink like a '04 or '05 will.) Yet even when it's not a huge issue, the server or wine steward is still obligated to display the bottle and inform you that the restaurant is out of the vintage that was listed on its wine list.
Then you, the diner, have a decision to make: accept the variation in vintage, or select another bottle from the wine list.
Most restaurants do this, but I still encounter many places whose staff are either too lazy or too ignorant to understand that this is the appropriate thing to do.