From the Verge:
“It was a mistake,” Gates admits to an audience left laughing at his honesty. “We could have had a single button, but the guy who did the IBM keyboard design didn't wanna give us our single button.” David Bradley, an engineer who worked on the original IBM PC, invented the combination which was originally designed to reboot a PC. “I may have invented it, but Bill made it famous,” Bradley said in an interview previously, leaving Bill Gates looking rather awkward. To this day the combination still exists in Windows 8, allowing users to lock a machine or access the task manager. While Windows 8 defaults to a new login screen, it's still possible to use the traditional Control-Alt-Delete requirement and a number of businesses running on Windows XP and Windows 7 will still use it every day.
Was the mistake that bad, though? Can you imagine this? You're three hours into an intense trip down the Oregon Trail. Your wife Turd is dead of dysentery, your son AssFart long gone from cholera. You've crossed the Rockies but your food options are slim. You haven't had a vegetable in four months. But you press on, hunting ducks, when all of a sudden your hand slips, hits the “Off” key—and the game shuts down. AssFart and Turd died in vain.
Control-Alt-Delete: literally, a life-saver. A beautiful mistake.
[H/T: The Verge]