See url, also screenshot provided by a user: http://i.imgur.com/2Vsq8V8.png.
Not sure what caused it, but it shouldn't break Echo because the notification can't be formatted.
Version: unspecified
Severity: critical
URL: https://en.wikipedia.org/w/index.php?title=Wikipedia_talk:Flow&oldid=594065615#Flow_.2B_Echo_.3D_Error.3F