Stolen From Apple
The Macintosh development team was inspired to take a peculiar anti-pirating step after Apple's experience with Franklin Computer's cloning of the Apple II. A 32×32 icon was developed and placed in the ROM in such a way that it was very hard for someone reverse-engineering it to locate. This allowed Apple personnel who knew of it to give the suspected clone a command to print a certain memory address causing the icon to appear if the ROM had, in fact, been cloned.
It exists at memory location $40AD40.