If the receiving team catches the onside kick and starts and runs and fumble the ball it can be advanced. Muff rule, just like a dropped punt cannot be advanced neither can a onside kick unless the receiving team run and control the ball then fumbles.
A few years back I think it was Central playing PN-G and Central scored and went up by 2points and PN-G blocked the xtra point and run it back for 2points and tied the game. PN-G won in OT.