The Bait n Strap
When a man is doing a woman from behind, the woman sneakily puts on a strap on (preferably made of a tough plastic complete with coloring). As the man is about to thrust inward, the woman shifts her balance so the man falls to the bed/tabletop/elevator floor/wherever the mating is occuring, and then the woman jumps on his back as he is still utterly confused and begins to bang him relentlessly in the asshole.
One time in Las Vegas I was screwing this prostitute doggy dog style when she unexpectedly turned on me and pulled the bait n strap on me and then hit me with a role of salami.