In one of the Garfield strips, Jon is at a high-school reunion. He talks to a man named Gary Barker. If you know a bit about Garfield, you'd know that Gary Barker is Jim Davis' assistant that draws the blue-pencil products before they are inked and lettered.
In one comic strip (July 17, 1994), where Jon is locked outside by Garfield while exposing himself to the neighbothood wearing Bunnie Jammies, you can see a blond kid wearing a striped red T-shirt. That's Calvin from the Calvin and Hobbes comic strips,... more