It's not a myth, try it on your system: open Notepad and save something as con.txt. Windows will tell you that the file name “is reserved for its use”. CON isn't the only expression that carries that condition. There are also old references to serial ports, parallel ports, and auxiliary devices that never went anywhere. Why are they still there? The short answer is “backwards compatibility”, but YouTuber Tom Scott explores it a bit more in one of his latest videos...
The Challenge of Backwards Compatibility
Changes in any computing environment are particularly hard to process. We still remember users' complaints every time Facebook changed its interface, or the resistance to the Ribbon system in Office 2007, which basically threw years of habits and muscle memory out the window. Another recent example is COBOL, and the need to ‘recruit’ new programmers to maintain mainframes with over 30 years on their shoulders.
Finding the exact balance between novelty and familiarity is very difficult, and on more than one occasion requires accepting various compromises. For example, Windows still has elements that date back to the 3.x builds and its dependence on MS-DOS, but some things are even older. The most “mediatic” case (if you can call it that) is the name “CON”. Every time we try to use it to save a file or create a folder, Windows says it is a “reserved name”. Why?
Tom Scott explains that CON and the rest of the reserved names (the full list appears documented here) are references that allowed (and still allow) users to “talk” to certain devices and send information to them. Their exact origins take us back to the mid-1970s, an era governed by CP/M and the technical limitations of those humble computers.
Although our exploration of the topic is a bit deeper, Tom's video makes it more visual: reserved names exist to guarantee backwards compatibility, even though the physical interfaces involved are almost extinct (it's very rare to find a Windows 10 machine that still has a dedicated parallel port).
The 1900 Leap Year Bug in Excel
Tom also mentions a very special case, the famous “February 29, 1900 bug” in Microsoft Excel. The year 1900 was not a leap year, however, when Excel first came out for Windows in 1987 (two years earlier on Mac), its main competitor was Lotus 1-2-3... which carried that bug. Microsoft's decision, in the name of backwards compatibility with 1-2-3 spreadsheets, was to reproduce the bug in its software. The 'error' is still there 33 years later, and is part of the official specifications.
