During the fight between the orc, Lurtz, and Aragorn, the moment where Lurtz takes out the dagger Aragorn had stuck it in his leg, and then Lurtz threw it at him. That was an actual dagger. It was supposed to hit the tree behind Viggo Mortensen, but because of the bad capabilities with the orcs armor, he accidentally threw it towards Viggo. Luckily, Viggo was fast enough to deflect it. The scene was therefore improvised by that.