420 Method Failure / Enhance Your Calm
Meaning
A deprecated or unofficial status code with multiple meanings: originally proposed in a WebDAV context for "Method Failure" (used by Spring Framework), and famously used by the Twitter API to indicate rate limiting with the message "Enhance Your Calm."
When to Use
Not used in standardized HTTP. "Method Failure" was used in some WebDAV-related responses when a method failed (now largely replaced by standard 4xx codes). Twitter used 420 to signal the client is being rate-limited (which later changed to 429).
Example
A Spring application might have returned 420 Method Failure for a failed API action, and early versions of Twitter’s Search API returned 420 Enhance Your Calm when the client was making too many requests.
When Not to Use
Do not use 420 in new APIs. For rate limiting, use 429 Too Many Requests; for method failures or other errors, use appropriate 4xx/5xx codes. 420 is not an officially recognized code.
Source
Unofficial (Spring WebDAV proposal / Twitter API v1)