This article is based on a study that documented the daily work practice of an eighth-grade mathematics teacher and her colleagues at a secondary school in Shanghai when they taught the unit on geometric proofs. Four interrelated work activities were identified to highlight a pedagogy around homework errors. They form an activity system characterized by rich and timely feedback to students and collegial deliberations on what is behind the errors. Key ideas of activity theory were used to shed light on the process and roles of each activity. Important implications of this pedagogy were discussed in the form of four dilemmas faced by the current reform. (Contains 1 figure and 1 table.)