Ambiguity has been identified as a useful tool for designing ubicomp systems. In the design of safety critical systems, however, the expectations for a system are particularly high, and goals of the technology are rigidly defined. In this context is not clear if open ended systems can still be used as a tool for design. In this paper we present a detailed account of the design process of an indoor wayfinding support tool for firefighters, in which ambiguity played a central role in driving the construction of the system. Based on an ongoing work covering more than a year of close collaboration with a heterogeneous team of project partners, we present some implications of using ambiguity for designing ubiquitous computing solutions in a domain that traditionally requires more formal specifications for the construction of technology.