A primary motivation of the Dialog State Tracking Challenge (DSTC) is to allow for direct comparisons between alternative approaches to dialog state tracking. While results from DSTC 1 mention performance limitations, an examination of the errors made by dialog state trackers was not discussed in depth. For the new challenge, DSTC 2, this paper describes several techniques for examining the errors made by the dialog state trackers in order to refine our understanding of the limitations of various approaches to the tracking process. The results indicate that no one approach is universally superior, and that different approaches yield different error type distributions. Furthermore, the results show that a pairwise comparative analysis of tracker performance is a useful tool for identifying dialogs where differential behavior is observed. These dialogs can provide a data source for a more careful analysis of the source of errors.