The website MathMistakes.org, needs to exist. I am happy to report my discovery that it does.

I don't think there are many things better for solidifying understanding and banishing misconceptions than examining the one's own mistakes and those of others. Math Mistakes is a great little authentic collection of student mistakes with accompanying commentary. Looking through these is a great way to understand different ways of thinking about a problem and solidify one's own comprehension by articulating the logic behind the correct solutions.

I think some great assignments could be crafted by asking students to comment on and correct these mistakes.