In IE 6 or 7, form validation can trigger a Stack Overflow error in the browser.
This issues appears to be caused by the isDeepVisible method call. Refactoring this code so that isDeepVisible is a function which takes an element as an argument, as opposed to adding the method to the Element object, should resolve the problem.
IE has known compatibility issues with recursive calls to methods added to an object.
The problem is most prevelent when validating components which are nested in several divs.