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Webpage error details
User Agent: Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 5.1; Trident/4.0; .NET CLR 1.1.4322; .NET CLR 2.0.50727; .NET CLR 3.0.04506.30; .NET CLR 3.0.04506.648; .NET CLR 3.0.4506.2152; .NET CLR 3.5.30729)
Timestamp: Sat, 24 Oct 2009 20:27:54 UTC
Message: HTML Parsing Error: Unable to modify the parent container element before the child element is closed (KB927917)
<!--[if lte IE 6]></base><![endif]-->
Judging from the error message, I'd take a good look at the DOM tree and make sure all elements are properly closed.
Maybe vBulletin is inserting some spurious tags in addition to the ones defined in your template?
Apparently, doing an appendChild() to an element further down the tree is a sure-fire way to trigger this behavior. Suggestions include deferring the script execution or placing it in the immediate parent element.