These checkboxes call their own and each others click() method from their onchange callbacks. Click the first checkbox. This should not crash or hang. This also shows that click() is protected on per-element basis.
 
checkbox1 onchange enter
checkbox1 onchange enter
checkbox2 onchange enter
checkbox2 onchange exit
checkbox1 onchange exit
checkbox2 onchange enter
checkbox1 onchange enter
checkbox1 onchange exit
checkbox2 onchange exit
checkbox1 onchange exit