Despite the recognition of the importance of teachers' professional development, the exploration of teachers' Critical Incidents (CIs) as a type of professional development initiative has received insufficient attention in L2 teaching contexts. Motivated by the dearth of research in this area and building on complexity theory as its conceptual framework, this study aimed to unearth Iranian L2 teachers' CIs over a three-month course. The initiative was framed around dialogic interaction among the participants who shared their written CIs, followed by online group reflection over the CIs. Post-course interviews were also conducted with the teachers to explore their reflections on the course. The analysis of the data through grounded theory indicated the interconnection among various factors and their contributions to the varied nature of the CIs experienced by the teachers, particularly the nonlinear nature of the CIs and their interpretations in the cognitive structure of the teachers. Additionally, the teachers regarded the initiative as a positive experience, enhancing their awareness of classroom occurrences and boosting their ability to manage their pedagogical decisions. The study concludes with a number of implications for teacher education.