There is, of course, no such thing as a good mashup. They exist because they must, because instrumental tracks and acapellas float freely across the web, because Acid and Garage Band are ubiquitous.
Yet. Sometimes we find one that says something important. Thanks to the ostensible generosity of folks like Jeff Atwood, I get tipped off to works like the one I'm about to embed. Perhaps Jeff could start a site called "Music Production Horror".