[Lol_Lolovici]

> our API responses and the innumerable clients and scripts that use these methods aren't currently designed with this state in mind

Well, you could just return the original URL when it is already shortened and this way there is no new "state" to worry about.

[PanMan]

Offcouse that IS a new state. If I call bit.ly's shorten function, I expect a bit.ly link back, not the original URL. I can imagine, for example, not saving the bit.ly part in a db, only the identifier. That breaks (without proper checking) if the original URL is returned.