It probably has to do with the way the Validation Toolkit is referring to the form in client side validaitons. It uses the numeric index of the form like document.forms[0]. If you then add an include above it that includes a form it throws off the form reference so that it needs to become forms[1]. You may be able to do a find and replace "forms[0]" for "forms[1]" to update the form references so that it will work with a form above it.