No guessing. What was the lie? Here's the dialogue.
Suzaku: "Lelouch, it's me."
Lelouch: "Suzaku. Why are you calling me now?"
Suzaku: "Lelouch, are you at the school?"
Lelouch: "No, but I will be soon."
Suzaku: "I see. I called because there's something I want you to tell everyone for me."
Lelouch: "Yes, and what would that be?"
Suzaku: "The sky. Make sure no one looks up at the sky."
Lelouch: "What?"
Suzaku: "Lelouch, is there anyone you could ever hate so much that you'd actually want to kill them?"
Lelouch: "There is, yes."
Suzaku: "I used to feel that sort of thinking was unacceptable, that unless you followed the rules, killing someone was just murder. But now, it's hatred that's guiding me. I'm fighting to kill someone. I'm going to become a murderer in the skies of Tokyo, so please..."
Lelouch: "Embrace your hate. Just think of Euphie. I made my own mind up long ago. I have no intention of turning back."
Suzaku: "For Nunnally, you mean."
Lelouch: "Yes. I have to go now."
Suzaku: "Thank you, Lelouch."
Lelouch: "Forget about it. I mean, you and I are friends, after all."
Suzaku: "For the last seven years."
Lelouch: "Right. See you, then."
Suzaku: "Yeah, later."
Justice divine doth smite you with its hammer.