An imperfection of character
Everyone associated with your target is a weak point.
When Xander didn't look away, she began to suspect she'd found a weak point.
Their weak point lies in their necessary conservatism; they cannot advance and adapt themselves to changed circumstances, as either monarchy or democracy can.
So exact is the latter determination that, were there no weak point in the subsequent parts of the process, this method would give far the most certain result for the solar parallax.
Its weak point is that the apparent motion of the node depends partly upon the motion of the ecliptic, which cannot be determined with equal precision.