This study investigates the prevalence and causes of inconsistency in translation memories (TM) using a sequential explanatory mixed methods design. The initial quantitative phase introduces a novel method and typology for measuring and categorizing inconsistencies in TM data. The data are the product of professional computer-aided translation of software documentation. In the follow-on qualitative phase, interviewees compare the quantitative results with their professional experience of TMs. Their confirmation of the quantitative results improves the validity of the study. The interview data also increase the utility of the research, suggesting possible causes and solutions for inconsistency. Results are presented interactively, followed by a short discussion of the findings and their consequences.