502 Bad Gateway
Meaning
The server, while acting as a gateway or proxy, received an invalid response from the upstream server it accessed in attempting to fulfill the request.
When to Use
When a server is acting as a proxy or gateway (like an API gateway, reverse proxy, or load balancer) and the backend service returned an error or something invalid that prevents it from serving a proper response to the client.
Example
A reverse proxy tries to fetch data from an upstream service that crashes or returns malformed data; the proxy returns 502 Bad Gateway to the client because it got an invalid response from the upstream.
When Not to Use
Do not use if you are not acting as a proxy or gateway. For direct server errors, use 500 or appropriate code. Also, if the upstream response is valid but indicates an error (like a normal 500 from upstream), the gateway might just relay that code rather than turning it into 502.
Source
RFC 9110