Another thing to be aware of is that when using prepaid gift cards at restaurants, hotels/motels and gas stations, the card may be charged at a higher amount than the actual cost of the transaction. This is because the final cost may actually be higher than the initial transaction (ie., a tip, phone use at a hotel, unknown gallons when filling up). I have used a bunch of these cards and have found that each establishment is different when it comes to how much above the initial purchase price they run the card for. This can be embarrassing if you know you have enough for the meal but the card is rejected. I imagine it can work the other way around though too if, for example, you purchase $100 on a meal and you only have $110 on the card and the merchant does not run your card for enough and then you sign the receipt for an amount higher than what was on the card (let's say $120). By the time they realize this has happened, you are long gone. This can easily be an innocent mistake if you are not aware of the exact amount left on the card.
Furthermore, it usually takes about a week for the "extra" amount to be credited back to your card too. So don't expect those funds to be immediately available.