Saving Private Ryan vs. Finding Nemo
A technique employed by the late Stanley Kubrick was to juxtapose serious imagery with lighthearted music (remember in Dr. Strangelove when the world was nuked to the tune of "We'll Meet Again"?). This basic idea has resurfaced in the viral video world, with enterprising editors creating custom movie trailers that completely alter the tone of the original film, to great comedic effect. My favorite examples of this are the Shining trailer (a nod to Kubrick, perhaps?), that makes it look like a feel-good family drama, and the Sleepless in Seattle trailer that resembles Fatal Attraction.
Another example of this idea is the above mashup that takes visuals from Saving Private Ryan and inserts dialogue from Finding Nemo. It's very funny, and actually matches well! Weird and cool.