OotP is just the book for getting derailed by little side puzzles
Dumbledore did Mundungus a favor, getting him out of trouble.
+
Mundungus got into trouble in the Hog's Head twenty years ago, and Aberforth's still mad.
=
What did Mundungus do in the Hog's Head that made it necessary for Albus to "save" him from Aberforth? It would've been half a decade or so before Voldemort's fall... Mundungus is most notably a thief...
You know, given the Aberforth/Mundungus confrontation in HBP, I think we might actually find out more about this story in DH.
Edited to fix typo: Original version said "...more about this story in HBP." I think the ship has sailed on that one.