Just a passing thought: A mathematics textbook with a single exercise at the front of the book which reads,
Every proof in this book has one (deliberately introduced) technical error, and may also have inadvertent errors. Correct all proofs.
those of us that value the moore method (having students prove ALL the results of a course, as well as discover some of them for themselves) know the value in assigning exercises of the form: "prove the following or find a counterexample". assign enough of these, and the student reflexively will know that any "stated truth" is suspect until proven. those of you have not tried the moore method, don't knock it until you try it. you cover far less material, but the students understand it much better because they "own" it. erich friedman