Wario: Master of Disguise contains this background that is only used in a debug room, and appears to depict a scene in an abstract art style not found anywhere in the parts of the game intended to be seen by the player.
Main Blog | Twitter | Patreon | Small Findings | Source: WMOD (NA, DS)
Top: in Wario World, the Wario statues at the entrances to the worlds have plaques that feature illegible text.
Bottom: zooming in on the texture of the plaques, extracted from the game's files, we can see that they possibly read "The greatest treasure hunter", with "The greatest treasure" being roughly recognizable while "hunter" is an extrapolation of the fact that the word starts with what looks like an "h" and appears to be 6 letters long.
Main Blog | Twitter | Patreon | Small Findings | Source: WW (NA, GC)