In the 80's I wrote a program for the Macintosh that had a customer base of a few thousand users. The "about" dialog featured the program's desktop icon magnified 8 times and doing a simple animation, but if you clicked on that giant moving icon all the magnified bits would individually fall to the bottom of the dialog and eventually bounce away.

Over the roughly 10 year distribution period for that software, one user reported finding the Easter egg.