If you're in the web-native generation that I am, the McDLT might not ring a bell. But those a bit older may remember that the McDLT was a burger with lettuce, tomato, and mayonnaise that McDonald's offered as a response to the Whopper. Its advertising campaign: "Keep the hot side hot, and the cool side cool."
To refresh your collective memories, check out this commercial starring Jason Alexander. McDonald's discontinued the McDLT in the early nineties, attributing it to the changeover from polystyrene packaging to more eco-friendly materials. (The polystyrene, George, the polystyrene!)
According to McDonald's grillmaster Ken Forton, the magic really was in the temperature: "They wanted the heel [bottom bun] and the burger to be warm, but the crown [top bun], lettuce, and tomato to be cold. And you can understand that; who really wants to eat hot lettuce?"
The packaging of the sandwich thus played an integral role; a two-part container had one side housing the warm elements, and the other side housing the cold ones. The consumer would then put the two together.
Though a variant (the Big and Tasty) now appears on the menu, ultimately the McDLT was discontinued. Some, like McDonald's, blame the packaging; it's necessary to have that polystyrene to insulate both sides from each other, which today's paper wrapping wouldn't. Ken Forton offered another explanation: "There was a special heating-cooling machine that we had. It was like a rack heater, but cold on one side, and hot on the other. I think a lot of locations just used regular heaters, so customers only ever got warm burgers." If you can't deliver on your slogan, the McDLT is just a veggie-topped burger.
But really, should fast food serve a sandwich that can't be eaten right out of the packaging? Leaving the "last mile" to the customer may have been the fatal flaw. In any case, some perfect storm whisked the McDLT into the depths of obscurity—well, and YouTube.