Error 503 related to Varnish cache server

I'm seeing this error pop-up sporadically on the documentation part of the
website.

https://www.postgresql.org/docs/current/static/tutorial-arch.html

Error 503 Backend fetch failed

Backend fetch failed

Guru Meditation:

XID: 460806

Varnish cache server

https://www.postgresql.org/docs/devel/static/index.html

Error 503 Backend fetch failed

Backend fetch failed

Guru Meditation:

XID: 231830

Varnish cache server

https://www.postgresql.org/docs/9.6/static/tutorial-sql.html

Error 503 Backend fetch failed

Backend fetch failed

Guru Meditation:

XID: 232015

Varnish cache server

David J.