1. A method of inspecting a photomask, comprising: forming an integrated circuit (IC) design comprising a graphical representation of an integrated circuit layout; using an initial image rendering model to determine a plurality of initial mask defects comprising defects in a photomask formed from the IC design; performing a plurality of modified image rendering simulations on the IC design using a plurality of modified image rendering models that respectively emphasize different types of lithography errors to determine a plurality of modified mask defects; and comparing the plurality of initial mask defects with the plurality of modified mask defects to determine falsely identified mask defects associated with one of the different types of lithography errors; wherein a computer is used to form the IC design, to determine the plurality of initial mask defects, to perform the plurality of modified image rendering simulations, or to compare the plurality of initial mask defects with the plurality of modified mask defects.