This study examined processes at the core of teacher professional development (PD) experiences that might positively impact teacher learning and more specifically teacher change. Four processes were considered in the context of a PD program focused on student mathematical errors: analysis of students' mathematical misconceptions as a lever for changing practice; review and discussion of images of existing and innovative teaching practices; video-enhanced reflection and detailing of new practices; and cycles of examination/implementation and feedback/revision. Participants were four US elementary school teachers, whose engagement in the PD program was studied through video observations and interviews. Findings revealed several positive shifts in teachers' interests and classroom practices. Challenges involved teacher content knowledge and counteracting cultural beliefs and practices on the role of errors in learning. The conclusions discuss implications for PD scale-up.