© 2020, Ministry of Science and Technology, Taiwan.The purpose of this case study is to investigate the triggers of unexpected (contingent) moments during one in-service primary teacher’s teaching of the measurement of length and perimeter, to identify this teacher’s responses to these moments, and to examine how these responses change the flow of the measurement of length and perimeter instruction. Ten lessons were recorded: the triggers of the contingent moments during these lessons were analyzed according to Rowland, Thwaites and Jared’s (2015) framework and the teacher’s responses were analyzed according to Stockero and Van Zoest’s (Journal of Mathematics Teacher Education, 16(2), 125–147, 2013) response types. The findings of this study showed that the contingent moments in these lessons resulted from the students’ incorrect answers, questions, and ideas, and the availability of materials. Moreover, the analysis of the teacher’s responses indicated that there were differences in the teacher’s spontaneous responses during the lessons, both in terms of the different types and the depth of the given responses. In addition, the teacher’s way of responding affected the flow of the measurement of length and perimeter instruction.