Formed teams with a few people from university I wasn't really compatible with. I wasn't prepared well either and it was my first hackathon. We bounced about a few ideas and decided to build a simple Javascript game, a Python backend and Postgres dB, techs I was barely familiar with at the time. Wasn't involved/couldn't contribute in the design or help much with coding. It was a humiliating experience overall.
Changed teams and formed a team with like minded people 4 months later and was better prepared this time. Built an app for social innovation and came runners up at the hackathon!