I get to use this activity twice a year (power rule for differentiation and then for integration) and I love it both times. In groups of three, each student has a problem. Everyone does the first step on their problem, then fold over so that only their work is visible. Then the students pass the problems around and everyone does the next step and folds so only their work is visible etc until the problems are done. Then they open them up and see of they are correct. If not, they look through and find the mistakes, and then we collect the most prevalent mistakes on the board. Students definitely seem to learn a lot by seeing mistakes, especially in developing that math sense that makes crazy stuff seem so crazy to teachers actually seem crazy to students too.