I spent the last couple of days in California on family business, and I had a morally interesting situation on the way out.
On my way to the plane, I stopped at a kiosk and bought a bag of mixed nuts for the flight. As I went further down the terminal, I realized that I also wanted a bottle of water, so I stopped at another kiosk. Since I was carrying the nuts, I put them down on the register table to get my wallet, and the cashier thought I was buying the nuts. I hadn't kept the bag or the receipt from the kiosk, so I couldn't "prove" that I had already paid for the nuts.
After a moments thought, I bought the bag of nuts ($4) but swiped a bottle of water ($2).