<p>Policymakers often assume that strengthening teacher quality through the provision of qualification upgrade programmes and reform-aligned instructional materials leads to improved teaching quality and the enactment of reform pedagogy. However, in this paper, we present the case of John, an out-of-field mathematics teacher who completed a qualification upgrade programme, to demonstrate that teaching quality can be compromised even when these reform supports are offered and utilised. Drawing on lesson observations from two timepoints, we investigate an error that emerged in John’s teaching during his participation in the programme and explore how he addressed the same content one year later, after qualifying to teach mathematics. We show that John adapted his teaching to avoid making the same error; however, we contend that these adaptations shifted his practice away from reform pedagogy and likely introduced latent vulnerabilities in students’ learning. Using the Swiss Cheese Model, we illustrate how vulnerabilities in John’s teaching across the timepoints may relate to vulnerabilities in the reform-aligned instructional materials and the design of the qualification upgrade programme. We argue that although teacher qualification policy and reforms aim to enhance teaching quality, they can contain vulnerabilities that may trigger or fail to prevent teacher errors. This study contributes theoretical and conceptual framings and analytical tools to advance a systems-based approach to studying teacher errors.</p>