As a meta joke, one of the visitable locations in the PC version of Mario's Time Machine (absent from the NES and SNES versions) is the office of Software Toolworks, the development studio that made the game. Traveling there, Mario meets various employees of the studio, among them David Grenewetzki, seen in the screenshot, who was the lead of the Mario's Time Machine project.
In David Grenewetzki's office, three Mario-related items can be seen: a framed picture of the Mario is Missing box art, another framed version of Mario official art, and a unique Mario-themed mug.