Wicket.Event.addDomReadyEvent is broken in IE since 1.5.1. I identified
WICKET-4080 as the cause of the problem: onreadystatechange with readyState="loaded" is sometimes fired before the document is fully loaded, causing scripts that reference DOM nodes to fail.
See http://msdn.microsoft.com/en-us/library/ms534359%28v=vs.85%29.aspx for the order of the readystate events.
The attached quickstart reproduces the issue (you may need to F5 a few times as it does not happen reliably). The actual JS error is:
SCRIPT5007: The property "onkeydown" has no value: The object is null or undefined
wicket-autocomplete-ver-1317728550188.js, Line 91 Character 9