script.isValid needed when?

"John J. Barton" <[email protected]> Wed, 17 Oct 2007 22:16:52 -0700
Newsgroups gmane.comp.mozilla.devel.jsdebugger
Message-ID <[email protected]>
Just to give a break from the porn posts, does any one know under what 
circumstances I need to test script.isValid?  In particular if I cross 
xpcom can I assume any script arguments will not zapped?  There are a 
couple of FF crashes involving Firebug and script code, eg 
script.functionSource crashes on FF2.

John.