In The Hobbit, the dragon Smaug has a thick hide, described as impenetrable armor. However, he has one weakness: a single bare spot, located in his underbelly. In the books, this weakness is unveiled through Bilbo's flattery and Smaug's arrogance, from whence a little bird told Bard the Bowman of it. Bard then shoots down Smaug using his trusty bow and a black arrow which had been passed down to him:
Arrow! Black arrow! I have saved you to the last. You have never failed me and I have always recovered you. I had you from my father and he from of old. If ever you came from the forges of the true king under the Mountain, go now and speed well!
In the movie The Hobbit: Desolation of Smaug, however, the Black Arrow was a ballista bolt, with an appearance similar to the metal used by Sauron's own. The killing of Smaug is accomplished with the use of a Dwarven Windlance, a ballista expressly said to be designed for killing dragons when used in conjunction with a Windlance, and the flaw in Smaug's armor was said to be caused by a previous attempt on his life with that same setup, by Bard's forefather Girion.
As Desolation of Smaug had 4 screenplay writers -- 3 working over Del Toro's own original screenplay -- the question of where the idea for the Windlance originated has come up. Whose screenplay first brought the Windlance (and artillery-sized Black Arrow) into play?