If you feel silly about the particular example or doubt its provenance or application, there is also the New Cuyama sign adding up numbers in an even worse way, the classic riddle where the answer is "you're adding the wrong things," or Scott's latest post with similarly bad math being applied to public policy.

I'm particularly fond of the New Cuyama sign as an illustration of the principle because so much online discourse seems to involve using funny pictures to illustrate points. Your quote works well for a Less Wrong audience, as we seem to be mostly text-based here.