Spencer Haywood challenged the rules of the National Basketball Association by declaring himself eligible for the draft after his sophomore season in college.  The NBA refused to allow him to play (as the rule at the time was that a player was not NBA eligible until the college class graduated) so he went to the upstart American Basketball Association and promptly won the league MVP as a rookie.  As you can imagine, somebody from the NBA would covet a talent like this, which was occurred when Seattle signed him anyway and a lawsuit went all the way to Supreme Court.  The NBA settled and the Sonics had a star.