Why do restaurants bother offering a ''taste'' of wine at the table from a bottle sealed with a screwcap? Obviously, the wine is not going to be corked.
Cultural hangovers last longer than the real thing. Witness the modern table knife: unlike its pointier predecessors, it's designed to slay little more than a slice of lasagne. There's no need to place it on the table in a way that signals to your medieval table-mates that your dinner plans don't include mid-meal murder and pillaging. But even though we no longer carry all-purpose knives for eating, skinning rabbits and stabbing knavish fellow diners, we still make the gesture of laying table settings with the sharp edge of the knife pointing peacefully inwards, towards the plate.
I wouldn't expect the restaurant tasting ritual to disappear any time soon. Even if screwcap seals could guarantee fault-free wines - which they can't - I reckon we'd still be offered tasting at the table for a good few decades to come. It's what we've come to expect, along with the exhibiting of the bottle to ensure the wine and vintage are as ordered.
A wine sealed with a screwcap is indeed not going to be ''corked'' in the literal sense, but it is possible for it to be contaminated with compounds such as those that cause the chlorine, wet-cardboard smell too often found in bottles sealed with cork. When this happens it is likely to be at such low levels that unless you are an airport sniffer dog or a Master of Wine, your odds of picking it up are not high.
Bottled wines under any kind of seal can suffer a range of woes: being ''cooked'' if transported or stored at high temperatures and excess oxidation - perhaps because of damage to the seal - are just two examples.
Be aware that there can be a fine line between wine that is indisputably faulty and wine that you personally don't like. The former is a reason to send the wine back but the latter is not. If you're in doubt as to which it is - and if you're in the kind of restaurant that takes wine seriously - ask to have the sommelier try the wine. Say you're not sure about it, and is it meant to taste that way? Someone who knows wine - and who knows customer service - should be able to talk you through why the wine tastes the way it does, and replace the bottle if necessary.