We have a no quibbles returns policy here in On the Grapevine - if a bottle is faulty, we will replace it without question. People don't take the piss with this policy, generally if they bring back a bottle, there is something wrong with it.
Occasionally though, a bottle is returned with nothing wrong with it. Recently we have had an expensive bottle of white Burgundy and an expensive bottle of red Bordeaux returned (by 2 different customers). In both cases, we replaced the bottles and, when we tried them later, both allegedly faulty wines were perfect. This gives us something nice to drink that evening, but these replacements cost us money.
Personally, I think it is worth it as the customer leaves the shop happy (always our first priority) and, as I mentioned, the policy is not abused. Or should I taste with the customer and try to educate?