The other day I was pairing with a colleague on a confusing bug with an nginx deployment. One of our proxy_pass directives was configured to point to the hostname of an AWS Application Load Balancer (ALB). After behaving normally in production for a few hours the nginx server started throwing 502s for all requests to a certain location block. It is documented that nginx will not re-resolve hostnames for proxy_pass entries that are set to static string values, so the first thing we confirmed was that the proxy_pass directive in question was configured with a variable value.