Right before the battle on Hoth, Princess Leah explains to the fighter pilots that the ion cannon will fire 2 shots to clear out any star destroyers as the transports make their getaway. When the first transport takes off we see the woman giving the order to fire the ion cannon. It fires 2 shots. Shortly after that, 2 shots hit the star destroyer and the transport escapes. However, in-between the shots and the hits, we see the transport and two Rebel fighters flying up from the planet and four shots fly past them, not two.