Unintended consequences or the road to hell is paved with good intentions.

A classic example of unintended consequences illustrated in this Wall Street Journal article Affirmitive Action Backfires.
The study cited in the article runs exactly as predicted by Thomas Sowell in many of his writings over the last decade. The laudable intent of affirmitive action is not in dispute here. The important point is well stated in the article:

As a matter of consumer fairness, law school applicants -- regardless of race -- need to know the statistical likelihood that someone with their academic credentials will successfully graduate and pass the bar. Once informed, they can better decide whether to undertake the risk of attending that particular school, or any law school at all. If law schools are unwilling to undertake this simple reform, it should be mandated by law.