Can't See the Fortress for the Keys...

When I wrote the last post, I looked ahead for the next few from Action #251 to see if there were any more appearances of the small (or large) key to the Fortress of Solitude.  However, when I slowed down to actually read those stories, I found something rather amusing.  In Action #253, Superman cautions Jimmy Olsen:

Okay, Supes, mum's the word. Except, in Action #252, the issue published just one month before that panel:

DOH!