For active landslides, drained shear strength is often determined from either a forensic slope stability analysis or laboratory residual shear testing. Both analyses often yield different shear strength values, often attributed to the complex, micro-scale behavior of the sheared material. However, subsurface asperities, such as cobbles, boulders, discontinuities, steps, and other geologic anomalies, may also influence the effective resistance realized along a highly sheared slip surface, thus influencing slope stability, and consequently landslide forensics. In this study, the influence of subsurface irregularities on effective mobilized residual shear strength, ϕr, and shear resistance from basal asperities, ϕa, is considered for idealized geometry under plane strain and three-dimensional conditions. The results demonstrate that even a modest presence of subsurface asperities may impart significant shear resistance, effectively reducing the drained residual shear strength that yields equilibrium. These effects are amplified when three-dimensional asperities are considered. There are significant uncertainties towards actually quantifying the geometry of subsurface asperities; however, this study highlights the potential role that such anomalies may impart on shear strength calculations in slope stability.