This idea was just too good to share - it is called My Favorite No. I was shown this video a few months ago at a school meeting and have been implementing it ever since. The teacher in the video uses it as a warm-up, but I adapted it and use it as an exit slip for my 6th graders. Check out the video (or read my description after if you are short on time!!).
My Favorite No
The idea is to give each student an index card or post-it note. At the end of class, I give them one or two problems that relate to our learning goal for the day. After collecting their responses, I quickly sort through their answers (in front of the class), sorting them into a "Yes" pile (correct) and a "No" pile (incorrect). As I look through the "No" pile, I pick out my favorite no, without saying names of course! I try to pick a response that had lot's of correct parts to it, but maybe one common or key mistake. As a class, we talk about the problem and correct the mistake.
I have found this activity to be very engaging for students! They all pay attention because they want to see if their problem is picked as "My Favorite No!"