There's a case before the SCOTUS right now that was argued back in October that has to do with something similar, where "forgotten law" (the police officer allegedly forgot what the law was for a broken tail light and pulled the person over anyway) led to a traffic stop, which led to a cursory search of the vehicle, which led to the discovery of a substantial amount of cocaine (relatively speaking). While one might argue that the court could easily rule for the government in this case given that the defendant consented to the secondary search (perhaps out of ignorance of the law), what's troubling for me in this related case is the government's position outright: namely, that it doesn't matter if the police officer does or does not know the law as long as a stop is "reasonable." To me that sounds like a pretty awful precedent to set. My law professor loved to ask us, somewhat condescendingly and to paraphrase John Adams, "are we a Nation of men, or are we a Nation of laws?"

Indeed.

[1]http://www.oyez.org/cases/2010-2019/2014/2014_13_604