Bruce Alter reports that after installing Mac OS 8 he began to get Type 12 errors when opening certain files in avariety of programs (including ClarisWorks 4.0, MoviePlayer, and SimpleText). It seemed particularly linked to QuickTime movies. Indeed, he noted that the QuickTime VR extension would be X'd out at startup. Reinstalling the system software did not remedy this.
Bruce also forwarded me two postings to the comp.sys.mac.system newsgroup that described similar problems. For example, a posting from Tom Jenkins stated: "I couldn't get any QuickTime applications to run. I restarted and noticed that the QuickTime VR extension was crossed-out. If I tried to run any QuickTime application (MoviePlayer, even SimpleText), it would crash with a type 12 error."