This morning I stopped for gas at a gas station that is associated with a grocery store. Buy more groceries, save a few cents off each gallon pumped at their station. That sort of deal. I found a gas voucher in my coat from a grocery shopping trip that should have allowed 25 cents off each gallon, so I figured I might as well use it before it expired.
When I scanned the little barcode on the voucher, I noticed that the display only registered a 20-cent-per-gallon discount. I also noticed that it would let me pump only ~7.5 gallons instead of the 20 that the voucher was good for. Luckily, there was an attendant in the tiny shack for the gas station that early. I approached him and asked what was going on--why I wasn't getting my full discount.
Obviously, he didn't believe my claims and had to see things for himself. He scanned the voucher and saw exactly what I described. Confused, he scuttled off to his shack to investigate. He couldn't figure out the exact cause, but ultimately he decided that someone else also had the same code or something from their own shopping trip. He was kind enough to actually give me the cash value of the 25-cents-per-gallon discount right then and there, so that's cool.
Moral of the story: if you use such gas vouchers, be sure to check the displayed discount with what you see on the voucher. If you notice a discrepancy, maybe you'll be lucky enough to get the cash value like I did! What makes it even more exciting is that I rarely use the full "up to 20 gallons" part of the voucher before the expiration. Bonus!