In the event that a system does not satisfy a specification, a model checker will typically automatically produce a counterexample trace that shows a particular instance of the undesirable behavior. Unfortunately, the important steps that follow the discovery of a counterexample are generally not automated. The user must first decide if the counterexample shows genuinely erroneous behavior or is an artifact of improper specification or abstraction. In the event that the error is real, there remains the difficult task of understanding the error well enough to isolate and modify the faulty aspects of the system. This paper describes an automated approach for assisting users in understanding and isolating errors in ANSI C programs. The approach is based on distance metrics for program executions. Experimental results show that the power of the model checking engine can be used to provide assistance in understanding errors and to isolate faulty portions of the source code.