Starting Friday, when you've been rate limited the HTTP result code will be set to 420. From the official, unofficial HTTP spec 420 is defined as "Enhance your calm". This shouldn't cause anything to break but you never know. If an app is checking the HTTP status and blows up on anything but 200 it will need to be updated.
This is mainly for us to be able to trace though logs to track what and who is being rate limited.