Vader: "See-threepio, Artoo never told you what happened to your creator."
C-3PO: "He informed me sufficiently. He told me you killed him."
Vader: "No... I am your creator!"
C-3PO: "No... no... That's not true. That's impossible! The odds against you being my creator are..."
Vader: "Search your feelings, You know it to be true."
C-3PO: "Noooo!! You see, sir, shortly after the Jedi were wiped out, my memory was erased. So that could explain why I'm having difficulty verifying your statement."
I :3 the SW wiki.