Hall of Fame forward Rick Barry is the only player ever to lead the NCAA, NBA, and ABA in scoring.
I've heard this statistic quoted a few times before, but after doing a little digging into it I believe that it's technically incorrect (the best kind of incorrect!).
Basketball-Reference.com shows Rick Barry as the leading scorer for the NBA in the 1966-1967 season, when he averaged 35.6 PPG in 78 games played.
Barry's contract with the San Francisco Warriors ended at the conclusion of that season, and Barry planned to sign with the Oakland Oaks, an ABA team coached by his father-in-law. However, the Warriors had a reserve clause in their contract with Barry (common at the time), meaning that the team retained his rights even after his contract ended. The issue went to the courts (the judicial ones), where it was ruled that Barry had to sit out for a year, but then could play for the Oaks the subsequent year.
In the 1968-69 season Barry averaged 34.0 PPG, which was more than any other player in the ABA. But, he only played 35 games that season due to a knee injury. Now, I'm not sure how many games are required by the ABA to qualify for a statistical season leader, but in the NBA it's 58 games (presuming a full 82 game season). So by that measure, Barry did not lead the ABA in scoring that year. And Basketball-Reference.com does not have him as the scoring leader - it gives the honors to Larry Jones, who had 28.4 PPG that year in 75 games played.
So there you have it... Rick Barry certainly had more points per game than anyone in the ABA in the 1968-69 season, but he didn't play enough games to qualify as the scoring leader.
And I'll leave you with this thought... with Barry having to sit out a full year early in his career he lost out on scoring any points (obviously). Presuming he would have scored 2,000 or so points that season, today he's be right around the #10 spot for all time scorers. But because of that missed season he's currently #22 on the list...