Senator Palpatine alter ego being Darth Sidious. 9 year old me deciphered this when I saw his name and picture on a pog when Episode 1 was released.
That was never really a secret though. They literally put it on the VHS back cover description, and there was no attempt to hide that both characters were played by Ian McDiarmid. Just another thing that proves the prequels were meant to be seen after the OT.