Computer science has its threshold concepts as any academic discipline that students struggle to understand deeply. In this work, we report on using an active learning method called scientific debate to teach programming threshold concepts in large unplugged classes. This method involves students by having them defend their position with arguments and scales up with the number of students by leveraging collective intelligence. We present in detail how we apply scientific debate in a programming class. We also collect and discuss student exchanges during debates and gather feedback after the last debate. Students report they stay more focused and motivated during class with scientific debate compared to traditional transmissive lectures. They also indicate that they understood the goal of this new pedagogical contract. Our experience report could help programming teachers willing to replace/complement transmissive teaching with a methodology involving the students.