I have started a separate blog devoted to helping students learn to find mistakes in worked problems (their own, or someone else’s). If this is of interest, check it out:
7/17/11 Update: There can be great value in work that contains mistakes. Learning to catch your own mistakes is a critical life skill, as is learning to review other people’s work while seeking to understand it fully (the best way to do this is by looking for mistakes).
Along these lines, I came across an interesting blog posting by Kelly O’Shea. She came up with the idea of insisting that each group who is presenting try to sneak a mistake in their work past their peers. Brilliant!