Due to https://bugs.webkit.org/show_bug.cgi?id=16829 NPAPI plugins haven't been able to use NPN_SetException() to throw exceptions in WebKit. However, [WebScriptObject throwException:] functioned from inside NPAPI plugins and could be used as a workaround. In ToT WebKit neither of these methods work anymore effectively leaving NPAPI plugins with no way to throw JS exceptions. This bug blocks Gears since we rely heavily on this functionality.
<rdar://problem/6483515>
Related to: https://bugs.webkit.org/show_bug.cgi?id=18234
It looks like the http://trac.webkit.org/changeset/33979 (Merge squirrelfish branch into trunk.) is what broke this.
Fixed by http://trac.webkit.org/changeset/39912 *** This bug has been marked as a duplicate of 16829 ***