The choice: In 1983, Microsoft released MS-DOS 2.0, which included a directory hierarchy like Unix. Unlike Unix, however, which used the forward slash (/) to delimit the directory path, Microsoft used a backslash (\), because the forward slash was already being used to indicate command line options.
The resulting pain: The backslash in Unix and many other languages such as Perl and C, is used to escape (i.e., to interpret differently) the character that follows it, causing much pain (not to mention headaches) for programmers going back and forth between the forward and backslash worlds.
The quote: "In retrospect it was a terrible decision, but I'd probably have done the same at the time." Dave Lindbergh