This is an activity that I created for my students to find the mistake in the expression that has been simplified incorrectly. I used common errors when creating the solutions to help them see the mistakes commonly made.
I had the students work alone to solve the problems first, and then share with their groups. If there was any discrepancy, they looked in the answer folder on their tables to see the correct answer. I rotated around monitoring and helping as needed. It was terrific to see the discussions that were going on! This really worked well!