Most common mistakes are,too small of an undercut and not wide enough face.on large diameter stubs this leads to trying to wedge it over,even though you have it sawed up close holding very little wood.when you finally get it to fall after wedging it up 2 or 3 inches,and your undercut face isn't wide enough up and down,the stub will fall until the face closes up and will stop at that point and you will have to saw it the rest of the way off the stump.solution is,make your under cut half way or more through the diameter,and make your undercut vertical face wider than normal.learned this the hard way years ago. Retired old grouth timber cutter.