Usually you order a bottle of wine in a restaurant and the waiter/wine guy opens it and pours a little for you to taste.  Conventionally, you are not supposed to be deciding whether you made a good choice, just whether or not the wine is corked, i.e. spoiled due to a bottling mishap or bad handling.  In practice this itself requires a well-trained nose.

But in some restaurants, the sommelier moves first:  he tastes the wine and then tells you whether or not it is good.

Suspicions are not the only reason some people object to this practice. Others feel they are the best judges of whether a wine is flawed or not, and do not appreciate sommeliers appropriating their role.

We should notice though that it goes two ways.  There are two instances where the change of timing will matter.  First there is the case where the diner thinks the wine is bad but the sommelier does not.  Here the change of timing will lead to more people drinking wine that they would have rejected.  But that doesn’t mean they are worse off.  In fact, diners who are sufficiently convinced will still reject the wine and a sommelier whose primary goal is to keep the clientele happy will oblige.  But more often in these cases just knowing that an expert judges the wine to be drinkable will make it drinkable. On top of this psychological effect, the diner is better off because when he is uncertain he is spared the burden of sticking his neck out and suggesting that the wine may be spoiled.

But the reverse instance is by all accounts the more typical:  diners drinking corked bottles because they don’t feel confident enough to call in the wine guy.  I have heard from a master sommelier that about 10% of all bottles are corked!  Here the sommelier-moves-first regime is unambiguoulsy better for the customer because a faithful wine guy will reject the bottle for him.

Unless the incentive problem gets in the way.  Because if the sommelier is believed to be an expert acting in good faith, then he never lets you drink a corked bottle.  You rationally infer that any bottle he pours for you is not spoiled, and you accept it even if you don’t think it tastes so good.  But this leads to the Shady Sommelier Syndrome:  As long as he has the tiniest regard for the bottom line, he will shade his strategy at least a little bit, giving you bottles that he judges to be possibly, or maybe certainly just a little bit, corked.  You of course know this and now you are back to the old regime where, even after he moves first, you are still a little suspicious of the wine and now its your move.  And your bottle is already one sommelier-sip lighter.