When I started playing DnD, we treated leaving a threatened square as actually leaving the entire threat range. So moving from one threatened square to another on the same creature was never a problem.
Now recently my friend has been playing and he's a tad more versed in playing and says doing so incurs an AoO. I'm inclined to believe him since he's been doing this for years but I felt I should check for every ones sake at the table. Is he right?
Best Answer
He is correct. From Combat → Attacks of Opportunity → Provoking → Moving:
(emphasis mine)
The only requirement is that you leave a square that someone threatened; where you go to doesn’t even come up (and, in fact, since that attack of opportunity occurs before the provoking action completes, the attack is resolved in the original square and it may change your mind about where you go or prevent you from going anywhere, e.g. with a trip or the Stand Still feat).