What causes a Magento 500 Internal Server Error?

A Magento HTTP 500 Internal Server Error almost always means a PHP fatal error somewhere in the request. The real cause is written to var/log/exception.log, the most common ones are an exhausted memory_limit, a broken third-party module throwing Class … does not exist, a bad .htaccess rewrite rule, wrong file permissions on var/ or generated/, or a missing setup:di:compile. We start by reading the last lines of exception.log, that single file usually names the culprit.