While theorists from Johannes de Muris to Johannes Tinctoris insisted that imperfection and alteration could be at work at multiple mensural levels (modus, tempus, prolation) simultaneously, requiring a complex balancing of different rules in context, in practice most mensural usage from the mid-fifteenth century to the early sixteenth century was both simpler and messier. In most cases, there is no more than one level at which musicians need to discern imperfection and alteration, and yet even so notational devices—dots, ligatures, rests, coloration, spacing—were optimized so that singers could rely on local notational context without having to analyze the overall mensural context. By extension, the standard signs of mensuration are not the only indications of mensural structure; sometimes the musical or notational context might lead us to override what we think the signs should mean. Drawing on theoretical and practical examples, this paper examines some of the conflicts and contradictions between mensural theory and notational and compositional practice. For example, irregular bar lengths bear witness to the mensurations holding less force as a structural unifier. Confusion between the concepts of mensuration and proportion lead to instances where a single sign could be followed by music in different mensurations, or a single mensuration could follow multiple signs (or none). Overall, I argue for a more context-dependent understanding of rhythm in fifteenth- and early sixteenth-century music.