While reading up on Fastly's downtime today, I actually found the explanation for this...
The error message comes from a type of caching software called
Varnish. It's open-source and widely used by a lot of websites. Fastly also uses Varnish in their content delivery service, which acts as a cache between a site's servers and its users.
The actual Varnish error message is "Meditation" (the project's logo is a meditating rabbit), but Fastly engineers changed the error message to "Mediation" in their Varnish servers so they could quickly tell if the problem was with their infrastructure or their customer's.