The issue where Clark visits Luthor in prison, especially:
Lex Luthor: (to Clark, who's taking notes) The inimitable Lex Luthor opened the floor and shook hands with a baboon in a Superman suit... Next panel... Lex:(shaking a baboon's hand) Evening, Leopold. Leopold: Raark.
The fact that the Fortress of Solitude has a little welcome mat on the front door.
With a key hidden under it. A key that weighs half a million (500,000) tons!
Possibly a Mythology Gag, referring to the giant "airline marker" key to the giant door back in the Silver Age continuity.