What does HTTP status 420 (Method Failure) mean?

HTTP_STATUS_METHOD_FAILURE

Older Spring Framework APIs exposed 420 as METHOD_FAILURE. It is a framework-specific historical value, so its actual meaning depends on the application that returned it rather than on HTTP itself.

Inspect the server framework version, controller or filter that created the response, and its response body. Clients should not assign a universal retry or authentication meaning to code 420.

The historical Spring constant does not give 420 portable semantics on the public web. If this value appears in an application today, locate the framework or application code that emitted it and read the response body. Standard clients should not assume that another service using 420 means the same kind of method failure.

Spring Framework 3.2 HttpStatus documentation