The error arose when Microsoft's own programmers forgot to include a single line of code that would have automatically triggered the "browser choice" program on versions of Windows 7 running its first major update, called Service Pack 1 (SP1).

A source close to Microsoft explained: "It was a single line in the code that triggered the browser choice program. It had a list of versions of Windows to test against: if the version was found in that list, the program would run. They didn't include Service Pack 1, which is effectively a different version of Windows, in that list. And so the program didn't run."