In Ennies Lobby arc when Luffy and his crew members trying to save Robin, Luffy order Sogeking (Usopp) to burn World Government's flag.
How can Luffy know that Sogeking can make something like flamethrower to burn the flag? He only know that Usopp that can burn the flag.
In that time, if I'm not mistaken, Luffy doesn't know about Sogeking's power, he also seems never notice Sogeking is Usopp before and even after the arc.