With today's conkeror (which has the dump_obj_r bugfix), use something like this in your .conkerorrc to
test dump_obj_r:

interactive("describe-buflist", "Show the contents of the buffer list",
            function (I) { dumpln(dump_obj_r(I.window.buffers,"The buffers:","---->",0)); } 
			 );
define_key(content_buffer_normal_keymap, "f11", "describe-buflist");
MAX_DUMP_DEPTH = 3; //otherwise it doesn't recurse

I get when pressing f11:
NS_ERROR_DOM_INVALID_STATE_ERR: An attempt was made to use an object that is not, or is no longer, usable
dump_obj_r()@chrome://conkeror-modules/content/debug.js:18
dump_obj_r()@chrome://conkeror-modules/content/debug.js:26
dump_obj_r()@chrome://conkeror-modules/content/debug.js:26
dump_obj_r()@chrome://conkeror-modules/content/debug.js:26

The error points to the line:
        for (var item in obj)

And the last objs being tested are:
[object nsXPCComponents]
[xpconnect wrapped nsIBrowserDOMWindow]
[object ChromeWindow]
[object ChromeWindow]
[object BarProp]
[object ChromeWindow]
[object OfflineResourceList]

OfflineResourceList is effectively the one which cannot be accessed with „for (var a in obj)“.

I found this explanation:
 http://www.archivum.info/mozilla.dev.apps.firefox/2008-06/msg00454.html
In that case it was a Gecko bug: https://bugzilla.mozilla.org/show_bug.cgi?id=422264