As computational thinking (CT) is new to teachers, they need support on how CT integration will work. This study examines the use of a hypothetical learning trajectory (HLT) to support teachers in integrating computational thinking into mathematics lessons utilizing GeoGebra. By employing the educational design research (EDR) methodology, the researchers, in collaboration with mathematics teachers and an educator, developed Math+CT activities in GeoGebra as part of an HLT. The HLT was meant to help teachers predict students' responses and facilitate learning. A pilot experiment was conducted with a small group of students to see how and why the designed activities work. Therefore, we collected data from students' responses (GeoGebra scripts and answers) and computer screen recordings. The HLT is triangulated by comparing the predicted responses to the actual responses. The pilot experiment helped to improve the lessons and the HLT for future experimentation. Considering students' possible mistakes in debugging would prepare teachers to anticipate and facilitate the contingency situation.