The poster for this movie features two snakes wrapped around a plane in a double helix style, with the film title across the middle, However some posters feature the same snakes/planes but with the title underneath. Take a closer look at this second type of poster and match each snake's head to its tail. if you match the right snaked head to the left tail the other snake head doesn't match up to the tail. Try this in any combination you like, ie. left head/left tail or right head/left tail. You will always be left with a head not matching a tail.

Take a look at the button the flight attendant presses when he puts the snake in the microwave. It's conveniently labelled "Snake".