This is a timely topic.
I just returned from Disney, where I used GCs to pay for our dining and souvenirs. I had two cards with balances of about $500 each. The cards were registered online, and were labeled on front with a sharpie to identify them. I also took along a couple of zero balance cards, also registered, in case one of the loaded cards was lost.
Tonight, while reviewing our receipts against the transaction history on disneygiftcard.com, I noticed an unusually large charge immediately following our Biergarten bill on 12-10. Usually you see your restaurant charge, then a separate smaller charge for the written-in tip. This time the second charge was NOT small.
I usually tip about 20% for decent/average service, but always adjust the cents so that the total bill ends in an even dollar amount. For our bill, that tip amount would have been $30.44. However, the amount charged to my gift card was $92.44. Because the charge ends in .44, and there was no charge for the correct amount, it is almost certainly the tip from the Biergarten. I would NEVER tip that much on a $150 tab. I had made a some other purchases that evening, so by the time the overcharge hit, it escaped my notice that the wrong amount had been taken.
I still have my copy of the restaurant bill and will be calling tomorrow to see if the store copy can be tracked down and checked for accuracy. Not sure what recourse there may be, since I used a gift card instead of a credit card, but you can bet they will be hearing from me.
In years of using gift cards at WDW, this is the first time this has happened to me. In the future, charging to a magic band then paying the balance at the front desk every day with gift cards may be the way to go. A bit of a hassle, but overcharges should be a lot easier to spot and dispute that way.