How does a 502 error differ from other common HTTP error codes like 503 (Service Unavailable) or 500 (Internal Server Error)?

A 502 Bad Gateway specifically indicates a problem with a server acting as a gateway or proxy failing to receive a valid response from an upstream server. A 503 (Service Unavailable) means the server is temporarily unable to handle the request (often due to maintenance or overload). A 500 (Internal Server Error) indicates a general error on the server, suggesting a problem with the server's code or configuration. A 502 implies a communication breakdown *between* servers, while the others point to issues either with the target server's availability or with the target server's internal processes.