The dramatic increase of online shopping has fanned the flames of competition between shipping companies. DHL figured the best way to advertise the speediness of their shipping services, was to trick its competitors into doing it for them: with the help of heat-sensitive ink.
The prank was simple and brilliant. DHL hired companies like UPS and TNT to deliver large black boxes to addresses that were particularly difficult to find. But the boxes weren't actually black. Instead, they were covered with a ink that appeared black when the boxes were chilled to sub-zero temperatures at the time of pickup. But as they warmed up in the back of delivery trucks, the thermally-activated black ink faded revealing a large message on the side of the boxes boasting that: "DHL is faster".
The deliverymen had no choice but to deliver the trojan packages, which were big enough to be incredibly awkward and extremely noticeable to anyone nearby. Well played, DHL. Well played. [YouTube via Tastefully Offensive]