Math Mistakes

I recently read about the blog Math Mistakes where you can submit a math mistake that was made on a math problem and then people can submit ideas on why the mistake was made and how to help the student!  I think this is a great teaching tool!!  I then read on another blog (can't remember which one) about an activity where students work a problem on a white board and make a mistake on purpose, then students do a gallery walk of the problems and have to write about where the mistake is made and how to fix it.  I love it!!  I am going to try this soon in my classes!