From a design point of view, there's not that many other events in the world on this scale. (Kumbh Mela comes to mind).

There's millions of pilgrims who are there for several days, in a pretty hot part of the world. They thing they are obliged to do was something determined a long time ago, and it has to be done at a specific place and time. There's old people, young people, people with handicaps, people who speak different languages. And then there's the ordinary stuff like feeding them, sanitation, etc,

That being said, you do have the experience of previous years to draw upon, and the host country certainly has resources.