This paper presents and discusses the use of serendipity in the design and use of experimental urban mapping tools and practices. We address the issue of error in design processes by exploring the role of serendipity in an experimental cultural mapping activity enabled by an iphone app of our own design. Our approach integrates aspects of chance and arbitrariness, and thus an alterative to dominant urban mapping methodologies. Also, our mapping approach contributes to a critique of digitally based forms of knowledge and functionalist dimension of locative app development and design. The uses of emergent, accidental and vagarious discovery is studied through as series of mapping instantiations to annotate urban transect walks in several Arctic cities. Our experiments show that issues of serendipity are productive in terms of adding new dimensions of creativity to practices of mapping, i.e. they open for new ways of looking, annotating/building knowledge and producing meaning in rapidly evolving Arctic communities. The affordances of the app and mapping approach make for creative readings of, and learning about, the city itself, and we address serendipity by discussing how it is embedded in the cultural use of the urban mapping tool. We discuss how the use the mapping approach can be understood in terms of serendipity and, finally, in what ways serendipity operates and informs notions and practices of transdisciplinary, participative and situated urban mapping.