The voluntary use of private device by employees without formal approval of the IT department, commonly termed Shadow IT, is an increasingly widespread phenomenon. In this paper, we study the role of private smartphones (and related applications like WhatsApp) in knowledge-intensive practices in the manufacturing domain. With an in-depth case study based on data gained from observations and interviews, we are able to empirically illustrate why workers use their private smartphones (contrary to company guidelines) and how they find significant gains of productivity by using the ‘forbidden’ applications. Our study contributes to knowledge management research by showing how private IT use can change existing knowledge management practices. At the same time, we are able to give rich insights into the rise of Shadow IT in a manufacturing context which takes place in a self-organised way without knowledge of the management. This enables us to take a step towards a knowledge management strategy perspective on Shadow IT.