Diffusion of comptency management tools allow to analyze how they are used by employees (Gilbert, 2003). This article shows that the analysis of unexpected uses is a way to better understand the dynamics of those tools. Data collected in a petrochemical company that pays the competencies for 10 years show that a management tool is consituted by heterogeneous elements that are articulated. The unexpected uses weak this articulation. Those uses appear therefore as an explanation of the trajectory of this management tool.