The detection and analysis of misunderstandings are crucial aspects of discourse analysis, and presuppose a twofold investigation of their structure. First, misunderstandings need to be identified and, more importantly, justified. For this reason, a classification of the types and force of evidence of a misunderstanding is needed. Second, misunderstandings reveal differences in the interlocutors’ interpretations of an utterance, which can be examined by considering the presumptions that they use in their interpretation. This paper proposes a functional approach to misunderstandings grounded on presumptive reasoning and types of presumptions, in which incompatible interpretations or interpretative failures are examined as defaults of the underlying interpretative reasoning, caused by overlooked evidence or conflicting presumptions. Moreover, it advances a classification of the types and the probative weights of the evidence that can be used to detect misunderstandings. The proposed methodology and its implications are illustrated through the analysis of doctor–patient communication in diabetes care.