While the nature of Unintended Consequences may perhaps be agreed upon, the determination as to when the term is to be applied is less certain.
Columbia Professor Michael Dorf has a different perspective on the actual root causes of the apparent consequences of the ADA laws cited by Freakonomics in their post Toward a Better Understanding of the Law of Unintended Consequences. He quotes Washington University of St. Louis Law Prof. Sam Bagenstos from a 2004 article in the Berkeley Journal of Employment and Labor Law:
In particular, I find quite plausible the argument that the 1990-1991 recession pushed an unusually large number of people with disabilities out of the workforce and onto the SSDI rolls--an argument pressed by the economists John Bound, Timothy Waidmann, David Autor, and Mark Duggan--though it is difficult empirically to disentangle that phenomenon from the effects of the ADA. Moreover, whatever the ADA's short-term effects, it seems likely that the statute's net long-term effects on employment or people with disabilities will be positive.Professor Dorf goes on then to say:
Dubner and Levitt could be forgiven for not reading all of the relevant law review literature; they are after all, not legal academics or lawyers, and disciplinary boundaries are often substantial. But even if they were unaware of the Bagenstoss piece, surely they must know of the whole book on the subject that Bagenstoss was reviewing, as it is very much a work of economics.
In addition to the factors Bagenstoss cites, one might also note---though Dubner and Levitt fail to do so---that discriminating against disabled persons at the hiring phase is also illegal.
the concept of "unintended consequences" is so appealing that I think it's often applied to settings where the consequences actually were anticipated and intended, at least by some of the parties involved.
Although hindsight is supposed to be 20-20, it can be difficult to say after the fact what the relevant factors were and whether or not they were intentional. There are other issues now ongoing in the world where the question of consequences is once more being raised and the question of intention can be put forward now. However, even when intentions are known and supposedly for the best by all involved, the consequences can still be far from desirable.