Extensive research has focused on what constitutes good route directions, identifying qualities such as the logical sequential ordering, the inclusion of landmarks, and ergonomic ways of referring to turns as critical to delivering cognitively adequate instructions. In many cases, however, people are not actually provided with route directions adhering to these qualities. Yet, often people are still able to successfully navigate to the planned destinations, despite poor or even erroneous direction giving. In this paper, we introduce the concept of defensive wayfinding as the particular type of problem solving people undertake when presented with route directions incongruent with their experience of the environment. We present a systematic investigation of the incompatibilities that may occur between route descriptions and the environment. We note that the content of route directions is produced by the direction giver based on observations of the environment. We develop a classification of the impacts of uncertainty in these observations based on the theory of measurement scales of Stevens . We then relate uncertainty to its impact on route following and the ability of the wayfinder to detect problems during wayfinding. We conclude with a discussion of the impacts of common-sense expectations on the need to engage in defensive wayfinding.