Case in point - the Joker just became pregnant and gave birth in the latest issue of Joker: The Man Who Stopped Laughing. The twist in question comes in the backup feature included in issue #4 ...

And the Joker ... Man that walks these shores, and I've seen all of it. I'm the immortal person here. You are nothing. I let you believe it for a little while because I wanted you to come with me ...