Bitcoin will make for some very interesting cases, I'm sure. I know of at least one person who is selling a product and allowing customers to overpay in fiat. He then sends the overpaying customer a "refund" in BTC. I'm confident this practice won't fly if it ever comes under investigation but consider another, hypothetical situation:
Instead of bananas, I start selling custom printed t-shirts. Each shirt has a unique QR code printed on the front that, when scanned, reveals a private key to a funded BTC address. I can charge more than one would usually pay for a t-shirt because each one is unique, see.
If I wanted to really be sneaky, I could send
two shirts to each customer. One has the QR code the buyer specified upon ordering (a random message, a URL, etc.) and the other has the private key QR code (How did that get in there? My cousin must have accidentally packed a personal shirt of mine! Darn my nepotistic hiring practices. I guess I'll never learn.
).