I hate to add to speculation... but my 2 cents worth...
The classic error is blockage of the static ports (on the side of the aircraft). There have been cases of aircraft coming out of repaint with the static ports still masked over.
This results in erratic airspeed indications, and it has been known that the pressure bleeds out at altitude, and then on descent the pressure in the static ports stays low, resulting in erronously high airspeed indication and a resultant stall on approach.