Ratchet & Clank Future: A Crack in Time
October 27, 2009
During the Plumber's sole appearance in Clank's subconscious, he is seen searching through a pipe looking like a Warp Pipe, which he later drops Clank into to awaken him from his mind trip but not before a sound effect plays that is similar to mario going through a pipe; he refuses to go in because, in his words, "plumbers don't go diving down strange pipes all willy nilly. That'd be ridiculous!" Which is obviously a reference to Mario.
The creators of Ratchet & Clank didn't hold any right over Nintendo's Mario, but they only included a minor reference to the series.