Rock slopes and colluvial slopes are common natural slopes in mountain areas. For simplicity, the slope stability analysis is often performed by two-dimensional limit equilibrium methods in terms of circular slide surfaces, which are not usually found in rock and colluvial slopes. This paper analyzes two cases of slope failure using the program SLOPE/W. The results show that we need to take into account weak planes or potential slide surfaces in order to evaluate the stability of rock slopes. For colluvial slopes, the usage of bedrock model or the assumption of a fictitious weak layer at the interface between colluviums and bedrock is a reasonable way for stability analysis to reflect common noncircular slides in colluvial slopes.