This is not only for Gathering visitors but anyone from abroad planning to use their Visa card.
This came up on another forum posted by a Canadian visitor to Scotland. When buying or paying for something using Visa, if the vendor makes a mistake and processes a refund the nice people at Visa treat it as if you were paying foreign currency into your account with them. Because they give a much poorer exchange rate to buy currency off you (funds in) than when they sell it to you (funds out) you are likely to find that the refund is actually a lot less than it should have been. So check your statement carefully, make sure you keep any documentation, and take it up with Visa right away. So be warned.