If you go to mootools.net in IE9, open up the developer tools, go to the console and enter the string "document.contains($('header'))' you'll get an error "Out of stack space".
This is a Slick issue.
I've looked into this. The problem is that it checks if document.documentElement exists and is the native contains method.
IE however does not have document.contains. So MooTools will set it with a Slick.contains wrapper. Now when you call document.contains it will result in an infinite recursion.
Probably an extra check should be added if document.contains is a native method.
This is fixed in Slick now.