The Emperor: Everything that has transpired has done so according to my design. Your friends up there on the Sanctuary Moon are walking into a trap. As is your Rebel fleet! It was I who allowed the Alliance to know the location of the shield generator. It is quite safe from your pitiful little band. An entire legion of my best troops awaits them. Oh...I'm afraid the deflector shield will be quite operational when your friends arrive.
Luke: Well, if that's all true, why didn't you wait until the Death Star II was nearer completion before you set up this 'trap'? And why didn't you give a fake location for the shield generator, or at least have a back up generator?
OK, Luke didn't say any of that, but he should have.