What teachers attend to, how they make sense of, and respond to critical incidents in the classroom are important for improving teaching. However, seeing and understanding important features of critical incidents can be difficult. In this paper, I propose a notion of productive noticing, which I used to analyse a case study of what teachers noticed about a critical incident that happened during a research lesson. Findings suggest productive noticing can help teachers to focus on important mathematical aspects of critical incidents, and understand how these can lead them to refine their teaching practices.