The Heroku Routing team does a lot of work with Erlang, both in terms of development and maintenance, to make sure the platform scales smoothly as it continues to grow.
Over time we've learned some hard-earned lessons about making systems that can scale with some amounts of reliability (or rather, we've definitely learned what doesn't work), and about what kind of operational work we may expect to have to do in anger.
This kind of knowledge usually remains embedded within the teams that develop it, and tends to die when individuals leave or change roles. When new members join the team, it gets transmitted informally, over incident simulations, code reviews, and other similar...