Just solved a bug in my client's application that was supposedly "caused by the switch to #elixir." After much debugging, I figured out the cause:

There was a race between their front end JS code and their backend code. If the backend code finished preparing some data before the UI was fully initialized, the frontend would barf, breaking the UI. This never happens in the pre-Elixir version of the code, because the old API never returned answer quickly enough. Task failed successfully, I guess?