Mosaic errors of immunity (MEI) encompass a group of immune disorders caused by somatic or gonosomal gene variants affecting hematopoiesis and immune function. Although the causal role of mosaicism in monogenic immune disorders has been recognized for over two decades, our understanding of their pathogenesis, genotype–phenotype correlation and clonal evolution remains poor. In this review, we synthesize shared and distinct molecular determinants from the currently recognized MEI and provide a mechanistic framework for future research. Exploring the implications of mosaic genetic variation in patients with unexplained immune disorders could uncover novel, actionable genetic disorders. Moreover, the study of these rare ‘experiments of nature’ may shed light on cell-specific immune pathways, non-malignant clonal dynamics, and mosaic disorders more broadly.