Last week after we learned more ways to notate a diagram to represent various things (midpoint, bisecting, ...), I handed out the following activity in class.http://www.blogger.com/img/blank.gif
I walked around a bit until I saw that a couple of problems were done and searched for a mixture of 3 to 4 papers that had mistakes of varying sorts and possibly some correct answers. Then I made sure to thank the students in advance for providing us with a learning opportunity and discussed how you can learn way more from searching for errors than from getting something right the first time.
I put problem #1 on the document camera and made sure that each student's name was not evident, then I asked the class to discuss with each other what were the correct things on the diagram shown and what were things that could be improved or corrected. I tried to use positive language and kept promoting it as a learning opportunity for ALL involved (which, obviously I think it is, but teens being teens....).
We went through a frew #1's, then did the same with #2. Then after I passed back the papers, I repeated the process with a few problems at a time as the need arose.
We especially had good conversations about #4 vs. #8. In number 8 many students wanted to make "T" be 2 different points. In #4, some students thought that if the angles had the same angle measure, then they were "the same angle".