HTTP 520 – Web Server Returned An Unknown Error
(Cloudflare) The origin server returned an empty, unexpected, or otherwise unrecognised response that doesn't match any other 5xx code.
Common use case
Cloudflare's catch-all when the origin server does something Cloudflare can't categorise into a standard 5xx, e.g. sending a corrupted or non-HTTP response.
History
A Cloudflare-specific extension, not part of any official HTTP specification.
Did you know?
It is effectively Cloudflare's version of "500-but-worse": the origin didn't even manage to send a coherent error, let alone a real response.