The error was FATAL: sorry, too many clients already , thrown by a Postgres instance that, by every dashboard we had, was using 40% of its CPU and half its RAM. Plenty of headroom. It rejected the connection anyway, which is the specific kind of outage that makes people distrust their own monitoring. The number nobody had looked at max_connections was set to 100, the Postgres default nobody changes unless something forces the question. We weren't near it on any given request —...