From theregister.com
More than 30 years before the xz backdoor became the near disaster of the week, an intern tried to sneak some unexpected code into MS-DOS. Not a backdoor, but potentially a bit silly.
At the end of a video in which former Microsoft engineer Dave Plummer attempts to explain the workings of the xz backdoor and how it came to be, a nugget was dropped about an Easter egg that almost made it into MS-DOS. It is not a backdoor but a cautionary tale regarding checked-in code and the benefits of code reviews.
Plummer’s story goes back to 1993, the final days of MS-DOS. He was an intern at the time, tasked with working on the disk compression technology in version 6.2 of the operating system. He shared an office with another intern who clearly had too much time on his hands.
“He was working on the copy command,” explained Plummer, “and he took the opportunity to check in – not a backdoor per se – but a special command line switch.”