Stress-inversion methods, used by structural geologists to estimate a regional stress tensor from populations of faults that contain slickenlines, rely on the basic assumptions that (1) the regional stress field is homogeneous in space and time, and (2) slip on each fault plane occurs in the direction of resolved regional shear stress. These assumptions ignore the effects of fault shape, the earth's surface, frictional anisotropy, and the proximity to nearby faults, any of which could result in a significant difference between the direction of resolved shear stress and the direction of fault slip. Mechanical modeling of common fault geometries using existing computer code DIS3D provides a means to evaluate the magnitude of this difference. The goal is to distinguish those geological circumstances under which existing stress-inversion techniques are reliable, from those that give unreliable answers. The results will provide new and important insights concerning fault behavior.