When on* attributes are updated in IE with event handler code that contains the 'this' variable the browser will fail to reference the targeted element (as expected). When window.execScript function is used to evaluate scripts the 'this' variable references the window object.
Also when 'return' statement is used in the event handler code IE will throw the following error:
Error: 'return' statement outside of function
Using 'return' statements in a event handler is a valid way of enabling or disabling the execution of default action in the browser.