McDonald's failed attempt at selling a burger with lots of veggies in their restaurants. The classic problem with this, of course, is the difference in temperatures among the ingredients. The beef patty must be hot, while the lettuce and tomato should be cold. (The bread, and any other condiments, are neutral in this equation.)
Back in 1985, the McDLT attempted to address this problem by being packaged in a two-chambered styrofoam box. On one side was the meat and cheese, while the other had the lettuce and tomato. The hot stays hot, and the cool stays cool. When you wanted to eat your burger, you would open the package and put the two halves together. Because you would eat the burger immediately thereafter, the condiments would come together in burger perfection.
Unfortunately, the whole container would often sit under a heat lamp behind the counter (this was before McDonald's started making things "Hot, Fresh, and Right To Order") so the toppings became limp and warm anyway. Furthermore, some restaurants tired of the voluminous packaging and just started making it like a regular burger. This led to the McDLT's death not too long after its introduction.
When I first started this node, I thought that the McDLT was some kind of BLT. That would make more sense, wouldn't it?