Lord of the Rings: The Two Towers - Disappearing Arrows
At Helmsdeep when all the uruk-hai are coming, every archer takes out an arrow on their bow ready to shoot it. After one guy accidentally shoots his arrow and kills one uruk-hai, they all take out another arrow and the other one they already took out disappeared.
The arrows don't dissappear. The only people that had nocked arrows before the first Uruk-Hai was shot by the old man were the Rohirrim. None of the elven archers had or nocked arrows or drawn their bows. The elves were the only ones who took out arrows at Aragorn command "Tangado a chadad!" ("Prepare to fire!"), not the Rohirrim. In fact, just before the old man shoots, there is a shot of Legolas still holding his bow undrawn while Gimili is jumping up and down. Sorry-nothing dissapeared this time.