When things appear to be going fine, then take a terrible turn for the worst. Based off of the style of writing/directing of filmmaker M. Night Shyamalan, a proponent of twist endings.
Dude, I was about to hook it up with this hot girlie, but then it turned out she was a dude. There were a lot of signs that suggested she was a dude, but I overlooked them; in hindsight it all makes sense. I just got Shyamalan'd.