I've never heard him claim that MS actually lost the source code to Word, only that they couldn't find the cause of a decades-old bug (related to selection and

I've never heard him claim that MS actually lost the source code to Word, only that they couldn't find the cause of a decades-old bug (related to selection and text-justification as I recall), even though it had thousands of bug reports, and that they spent years looking for it before giving up. The point was that the code is too big and complicated, even for some of the best engineers in the world.