Deterministic (conventional) kinematical approach based on the use of most observable discontinuity orientations, is the commonly employed method for the preliminary assessments of stability of slopes excavated in jointed rock masses. But, other possible discontinuity orientations distributed around the central clusters may exist and they may also contribute to failures. The effects of such discontinuities on instabilities are assessed by probabilistic kinematical approaches. In this study, the southern slope of the Himmetoğlu lignite open pit mine operated by the Turkish Coal Enterprises (TKİ) and subjected to instabilities causing interruption of coal production, is investigated by both kinematical approaches. The results derived from both methods are compared and evaluated for the whole slope and for a local planar failure occurred at its west. Besides, it is shown that wedge failures controlled by the intersection of faults and joints seem to be probable on bench faces. However, from the design point of view, determination of unsafe bench orientations within a risk interval seems to be more useful than introducing a single orientation value. For this purpose, on the basis of given confidence limits and variations in the orientations of bedding planes, probability graphs are suggested to estimate the orientations of unsafe bench faces in the southern slope along which planar failures are probable.