In this case study we discuss the dynamics that drive a free-of-charge project-based learning extension course. We discuss the lessons learned in the course, "Laboratory of Social Projects." The course aimed to teach project management skills to the participants. It was conducted from August to November of 2015, at Federal University of São Paulo (Unifesp), Osasco Campus, Brazil. The course had 72 participants (41 community members and 31 university students). The participants worked in teams of four members (on average) and developed 13 projects on behalf of eight NGOs that help people in need. In our research, we followed a mixed methods approach, using project reports, project blogs and the course management database as sources of information. We made use of system thinking analysis to reveal the dynamics that unfolded during the course. Our main findings are the following: 1) free-of-charge extension courses can be much more challenging to manage than traditional courses once the workload of the professors involved can be substantially higher than in similar regular courses; 2) the use of project-based learning techniques in extension courses can be very effective; 3) problems are created when the students drop out of the course.