This incident holded on for some minutes. Text of the error message:
503 Backend fetch failed
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Please provide the details below if you report this error to the system administrators via https://phabricator.miraheze.org:
Error 503 Backend fetch failed, forwarded for 126.96.36.199, 127.0.0.1 (Varnish XID 1377520) via cp1 at Fri, 07 Jul 2017 08:49:25 GMT.