Here's a quick list of common HTTP error codes along with brief descriptions:
400 Bad Request: The server cannot process the request due to client error (e.g., malformed request syntax).
401 Unauthorized: Authentication is required and has failed or has not yet been provided.
403 Forbidden: The server understands the request but refuses to authorize it.
404 Not Found: The requested resource could not be found on the server.
405 Method Not Allowed: The request method is known by the server but is not supported by the target resource.
408 Request Timeout: The server timed out waiting for the request.
409 Conflict: The request could not be completed due to a conflict with the current state of the resource.
410 Gone: The resource requested is no longer available and will not be available again.
413 Payload Too Large: The request entity is larger than the server is willing or able to process.
414 URI Too Long: The URI provided was too long for the server to process.
415 Unsupported Media Type: The request entity has a media type which the server or resource does not support.
429 Too Many Requests: The user has sent too many requests in a given amount of time ("rate limiting").
500 Internal Server Error: A generic error message when the server encounters an unexpected condition.
501 Not Implemented: The server does not support the functionality required to fulfill the request.
502 Bad Gateway: The server received an invalid response from the upstream server.
503 Service Unavailable: The server is currently unavailable (overloaded or down for maintenance).
504 Gateway Timeout: The server, while acting as a gateway or proxy, did not receive a timely response from the upstream server.
505 HTTP Version Not Supported: The server does not support the HTTP protocol version used in the request.
These codes help diagnose issues with HTTP requests and responses.