W.A.D.N.A.I is one of my best, most favorite acronyms. I came up with a few years ago in the face of... well.. a W.A.D.N.A.I. situation.
Definition:
W.A.D.N.A.I. - Works As Designed Not As Intended. (Pronounced Wad-Nye)
When the software you've written does exactly what it is supposed to do - it works as designed. But, it does not do what the end user (or the fool who wrote the specification). Does not work as intended.
It's a pretty easy to blunder into a WADNAI. Either the client did a lousy job of telling you what they really wanted, either because they didn't know or because you didn't get to the heart of the requirements.
The only advice I have from my own WADNAIs is: Don't accept all the blame, they helped you get here. But don't think you're going to get off scott free - you gotta fix it.