It took a while, but I did finally locate the wording I was looking for.
It is an NFHS rule:
7-4 Batter is out
Art. 13 After hitting or bunting a ball, the bat hits the ball a second time while the ball is on or over fair territory, or is on or over foul territory and, in the umpire's judgment, had a chance to become a fair ball.
EXCEPTION: If the batter is in the batter's box and the bat is in the batter's hands when the second contact occurs, it is a foul ball regardless of whether the ball was on or over fair or foul territory unless, in the umpire's judgment, the batter intentionally hit the ball a second time.
USA Softball has no such wording.
I remembered this odd wording, and initially thought it might apply to a batted ground ball that was foul and might become fair. Clearly, that is not the case as the above wording applies to a ball hit a second time.
The USA Softball wording of this situation seems a bit clearer to me, although there have still been questions about it. Perhaps there was a clarification on this a while back.