<p>Supplementary materials have become a central feature of the contemporary online publishing system, yet they remain largely overlooked by metascience research. I argue that supplementary materials should not be viewed as neutral side files passively supporting the main document, but rather as motive-laden objects capable of influencing the evaluation of scientific research. Specifically, the benefits of supplementary materials may depend on the motives shaping their content. While supplementary materials motivated by transparency may reduce publication bias and enhance readability, those driven by consistency motives risk becoming a new kind of file drawer. Indeed, these files offer optimal settings for spinning research outcomes by sorting information between the main document and supplementary files to fit one’s narrative, making information technically available but effectively invisibilised. Even if unintentional, and while distinct from questionable research practices such as selective reporting—where information is voluntarily omitted altogether—selectively presenting content between the main document and supplementary files has the potential to distort how research is perceived by stakeholders. I discuss the dual role of supplementary materials in promoting robust science and enabling spin, and how external constraints, such as word limits, may exacerbate these dynamics. Finally, I propose ways to limit the negative impact of supplementary materials through stronger justification, organisation, and control of information, and I consider opposing perspectives.</p>