If you mouseover CMSMenu items (in Kentico 3.1a) before the page is finished loading (so you have to do it quick), IE6 will throw an error. Not a big deal because drop-down menus still work after the error, but if you have debugging turned on it's annoying.

Fix:

~/CMSScripts/skmmenuup.js:352
from:
if (skm_subMenuIDs == "undefined") return false;
to:
if (typeof(skm_subMenuIDs) === "undefined") return false;

~/CMSScripts/skmmenu.js:346
from:
if (skm_subMenuIDs == "undefined") return false;
to:
if (typeof(skm_subMenuIDs) === 'undefined') return false;