This slip-up involves Legolas during the Battle of Helms Deep. When Legolas grabs the shield and uses it as a surf board down the stairs, he is also shooting arrows. The movie will pan away from Legolas for a moment and when it comes back to him if you zoom in X2 and slow it down you can see that there is one time that he reaches back and shoots the arrow, there is actually no arrow in his hand. How's an invisible arrow going to kill an Uruk-hai?