Keroro Gunsō episode Keroro And Natsumi Which Way to Heroes?!
September 08, 2006
In the twenty-third episode of season 3 of the anime (Episode #126), to demonstrate a new "Reality-avoiding" device, Kururu and Saburo transport themselves into a videogame which is clearly based on Super Mario Bros. Also, Kururu is dressed as Mario and Saburo is dressed as Luigi.
The creators of Keroro Gunsō didn't hold any right over Nintendo's Mario, but they only included a subtle reference to the game.