A notable example is the approach to soft drugs in the Netherlands. Despite being illegal, the public prosecutor has chosen not to enforce the law. To the point that many if not most think they're legal.
This situation presents a complex issue to me: it involves a small group of individuals (the prosecutor's office) effectively deciding to disregard the broader democratic process and the will of the voters. When such things happen, I believe they should be rare, pragmatic and temporary.
The 'selective enforcement' occurred because strict enforcement would be much more expensive than what anyone wanted -- yet a fanatical minority was able to play games in Congress to repeatedly block bipartisan deals for "comprehensive immigration reform" (under Bush, Obama, and Biden).