What’s the difference between a Magento 500 and a 503 error?

A 500 is a fatal PHP error during the request; a 503 Service Unavailable usually means the store is in maintenance mode (var/.maintenance.flag) or the generated/ code is missing/stale right after a deploy. A 503 with “Service Temporarily Unavailable” after deploy is the classic “forgot to run setup:di:compile / setup:static-content:deploy” signature. We confirm which one you have from the status code and the logs before touching anything.