Over the last ten years Agile Software Development (ASD) has received much attention from researchers and practitioners as an approach for dealing with change. However, the proper application area and the use of a mixed - agile and traditional, more plan-driven -approach are still much debated. In this paper, we report from a mission critical project that was considered agile by the involved staff, but which actually employed a mixed agile and plan-driven strategy. We introduce a framework, which allows for (1) descriptive analysis of the project, (2) its discussion against the agile values as presented in the agile manifesto, and (3) a comparison of findings to Complex Adaptive Systems (CAS) theory. We contribute to the debate with rich insight about: which work practices were applied in practice? which of the applied work practices were agile and/or which were more plan-driven in nature? and which of the applied practices fit with CAS theory and/or with a more plan-driven perspective? The analysis of our case shows that some of the agile practices were used in a way that supported both agile values and a traditional focus on processes, documentation, and planning. Moreover, certain traditional practices were in line with CAS theory, while some agile practices fit both CAS and traditional concepts. We suggest that to understand ASD in practice it is relevant to investigate how the applied practices are actually used in the particular case and that the agile manifesto and CAS theory are useful, complementary lenses for doing so.