I was working for a defense contractor in the late 80's. I had developed a prototype test driver for our system, that talked to the target system over an RS-232 interface.
This was back in the days of serial terminals, so no graphics.
I had put in an easter egg, that if you ran the program with my name as the sole argument, it displayed a little ASCII animation intro. The funny thing was that I had actually made a mistake on the first cut, and it scrolled in upside down. So I kept it, added "Oops, start over", and reran the animation properly.
It was originally for internal use only, so nobody cared. Eventually the marketeers saw some potential in it, and started selling it to DoD clients. They made me take the egg out at that point.