Following a series of events I won't recap here, this morning I launched Echofon for Mac to discover my password had been cleared out and instead of tweets Twitter was sending back a 401 Unauthorized response code.
I clicked on the error message, expecting Echofon to give me some way of fixing it, but instead was taken to the HTTP Response Codes and Errors page of Twitter's developer documentation. On that page I learned that rate limit errors are handled with the following code:
420 Enhance Your Calm: Returned by the Search and Trends API when you are being rate limited.
The 420 error, as far as I know, is not part of the official HTTP spec, though error 418 I'm A Teapot is. And this isn't the only case of a software company coloring outside the lines of HTTP: Apple uses the 402 Payment Required code (defined in the spec as "reserved for future use") to indicate when a MobileMe account is past due, while Microsoft added its own 450 Blocked by Windows Parental Controls status, I guess because none of the 6 or 7 existing error codes concerning controlling access to resources were specific enough.