What does a 500 / 503 error mean and can you fix it?

A 500 is a server-side crash; a 503 means the service is unavailable (often a stuck Magento maintenance flag after a failed deploy). Both are fixable, usually quickly. We read the server, PHP-FPM, and application logs to find the real cause, a fatal error, an out-of-memory, a bad config, or a half-finished deploy, then clear it and harden against a repeat. For the full diagnostic, see Magento 500 error fixing.