Back in the mid-late 1990s I worked for a software company producing anti-virus software, and scanning Word files for macro viruses was one of those things they had to do. However, they weren't able open Word 1.0 files, so they brought over some actual Microsoft employees from the US to help. For a relatively small (at the time) UK company that was actually quite a big thing and the only time I'm aware they did something like that. It was all a bit hush-hush, but the story I heard was that Microsoft had lost the original source code to read and write Word 1.0 format files (although they still had compiled binaries), so new code had to be reverse-engineered.