If I go to
http://pt.wikibooks.org/wiki/Especial:Prefer%C3%AAncias?uselang=qqx#mw-prefsection-gadgets
and enable this first group of gadgets:
[1] IEbug
[2] Edittools
[3] Correlatos
[4] ExpandAllOnPrint
[5] FeedbackHighlight
[6] rev-e-avisos
[7] ProgressLevel

or this second group of gadgets: [1], [2], [3], [4] and
[8] BookGadgets

and open the page
http://pt.wikibooks.org/w/index.php?title=Test&action=edit&debug=1&uselang=en

using IE9, I get the error

Line: 170

Error: Invalid argument.

The line 170 is this part of the definition of mw.util.addCSS:
s.styleSheet.cssText = text; // IE
http://svn.wikimedia.org/viewvc/mediawiki/trunk/phase3/resources/mediawiki/mediawiki.util.js?view=markup#l164

The error disapears if

• I disable any of the gadgets in the first group above (and keep the other 6 enabled) or
• I disable any of the gadgets in the second group above (and keep the other 4 enabled)

The code of [[MediaWiki:Gadget-IEbug.js]] is just
mw.util.addCSS( 'a { color: red; }' );
mw.util.addCSS( 'a { color: blue; }' );
and was created to try to find the cause of the bug, which was happening on
https://pt.wikibooks.org/wiki/MediaWiki:Gadget-Internet_Explorer.js
(which was copied from [[MediaWiki:Common.js]])

Version: unspecified
Severity: normal
URL: http://pt.wikibooks.org/w/index.php?title=Test&action=edit&debug=1&uselang=en