So without any particular introductions: we all know that Original Dungeons & Dragons (especially if we consider only the three basic booklets) is a game which needs quite a lot of rule supplementation if we want to cover most of typical in-game situations. And the rules which are present are not always that useful and you don’t need to be a genius designer to replace them.
The result is that after each campaing the referee ends up with a bunch of houserules. After few iterations, dozens of games played and few campaigns one may find that surprisingly the volume of houserules qualifies for a completely new game system.