A Happy Mac is the normal bootup icon of an Apple Macintosh running the classic Mac OS (not Mac OS X however). When you boot one up, it makes a beep noise, and the screen turns gray. If everything goes okay during the diagnostic tests, a happy Mac icon will appear, followed by the splash screen of whatever version of the Mac OS you are running. If the diagnostics fail, a Sad Mac icon will appear instead, signaling that there has been a problem in the bootup sequence.