An inspection system, and a computer program product that stores instructions for: obtaining an aerial image of an area of the mask; wherein the aerial image represents an expected image to be formed on a photoresist of an object during a lithography process that involves illuminating the area of the mask, by a lithography tool; wherein the photoresist has a printability threshold; wherein the lithography process exhibits a process window that introduces allowable changes at pixels of the expected image that do not exceed an intensity threshold; and searching for at least one weak point at the area of the mask, which is either a local extremum point of the aerial image spaced apart from the printability threshold by a distance that does not exceed the intensity threshold or a crossing point of the printability threshold and being of a slope that is below a predefined threshold.