If you encountered a 502 error getting to the forums over the past 12-ish hours, that was me.
I had the hosting company doing a BIOS upgrade on the web server last night, which required a reboot, and it appears that someone hit the CoG forum URL at exactly the right instance at the tail end of the reboot, just before the Discourse docker container was fully back online. The 502 bad gateway response was cached, and Varnish has been serving that since last night. It would have eventually fixed itself as the cache expired, but I’ve killed & restarted Varnish and that appears to have fixed the problem.
Thanks to @nabiki for the heads-up!
tl;dr - Caching. It’s always caching.