Here is a little worksheet I made for students to explore common algebra misconceptions. I'm planning to use it as part of a small-group activity, as a way to elicit discussion about algebra concepts.
Can you see why someone might think (Calvin in the last panel) that
n + n = 28
MacGregor, M., & Stacey, K. (1993). What is x? The Australian Mathematics Teacher, 49(4), 28-30.
and Calvin just forced his way onto the page (apologies to Bill Watterson!).