A prerequisite for mobile robot navigation is a reliable sensing mechanism. Laser detection and ranging sensors or LADARs are widely used in mobile robotics. When processing LADAR data for the purposes of feature extraction and/or data association, most previous work models such device as processing range data which follows a normal distribution. In this paper, it would be demonstrated that commonly used LADARs suffer from incorrect range readings at range discontinuities, which can have a much more detrimental effect on feature extraction or data association algorithms than random noise. LADARs with separated transmitter and receiver configuration can introduce a significant occlusion effect, as the reflected laser energy from the target can be partially occluded from the receiver. This paper would demonstrate that false range values can result from LADARs and that the occurrence of these values can be reliably predicted by monitoring the received signal strength. A useful design criterion for the optical separation of the transmitter and receiver is also derived for non-coaxial LADARs. The parameters, which are related to range errors, are quantified so that such errors can be reliably predicted.