Just to make it more interesting, the Supreme Court just upheld that cops can be excused for some reasonable mistakes in their interpretation of the law...
http://www.washingtonpost.com/politics/courts_law/court-rules-police-may-make-reasonable-mistakes-in-law/2014/12/15/b8b43cf6-8495-11e4-b9b7-b8632ae73d25_story.html
Yeah but when it happens to us they say ignorance of the law is no excuse.