Rigorous simulation of electromagnetic scattering and imaging are used to assess the tendency of nonplanar phase defects to print in optical lithography with alternating phase-shifting masks. The ideal model for phase defects, which assumes that the fields are transmitted with unaltered amplitude and a phase shift of 180 degrees, is proven inaccurate through simulation of isolated defects. The brightness and the phase are found to depend strongly not only on their height but on the lateral size as well. Data are also presented on a variety of defect shapes, sizes, heights, positions, and their impact when located near mask features. Linewidth variations due to lines/posts and scratches/holes are typically 30% larger and 50% smaller, respectively, than these predicted from an ideal geometrical mask model.