Casting directors having fun with reality:
The same actor who played Mark Zuckerberg became Lex Luthor.
The less famous brother in Entourage was played by Matt Dillon's less famous brother.
The perfect casting for Serge Gainsbourg was almost his daughter, Charlotte.
A character in The Rock was effectively James Bond. So they cast Sean Connery.
Evan Peters crossed realities and franchises to cameo in WandaVision.
In the TV show You, Chris D'Elia played a predatory sex abuser - apparently an industry open secret given he was then was accused of the same in real life.
Most audaciously (or obnoxiously?), Julia Roberts played a character in Ocean's Twelve who was a doppelganger for - Julia Roberts.
Sometimes you can't write it.
But you can cast it.