Dear Yahoo(!) Pipes team,
Why do your feed URLs return this error when you’re busy?
HTTP/1.1 999 Unable to process request at this time -- error 999
Not only are you using a nonexistent code, but there’s a perfectly valid one to represent what you mean.
HTTP/1.1 503 Service Unavailable: “The server is currently unable to handle the request due to a temporary overloading or maintenance of the server. The implication is that this is a temporary condition which will be alleviated after some delay.”
Instead, you report the bogus “999” status, so requesters following the HTTP spec (like the Tumblr feed crawler) have no idea what to do or tell the user.