Obligatory. The Mary Poppins trailer re-cut as a horror movie. Scary Mary.
http://www.youtube.com/watch?v=2T5_0AGdFic
(As an added bonus, the trailer for The Shining, re-cut as a romantic comedy.)
http://www.youtube.com/watch?v=KmkVWuP_sO0
Scary Mary is genius!