David Gay tried to save a Word 6 document as a Postscript file when running Mac OS 8. Instead, he got an ID=-50 error. He replicated this on three different Power Macs. Documents from other applications (such as Canvas) could save Postscript files without a problem. And Word could save documents as Postscript files when running Mac OS 7.6.1.
Update: Edgar Lazaro writes that "this problem is actually a problem between LaserWriter 8.4.x and Word 6.0/6.0.1. Downgrading to LaserWriter 8.3.x should solve the problem."