Generally I think QR codes are a lame stopgap technology. But this application by the Korean retailer Emart is fairly clever. They deployed 3D "Shadow QR codes" that only work during certain sunlight hours, lunchtime specifically. I could imagine this kinda thing being a fun clue in an alternate reality game. "In Seoul, retailer uses 3D QR codes and the sun to deliver discounts only during its quiet times" (Springwise, thanks David Steinberg!)