If you use ModalPopUp in an AJAX page, and in that modal popup you have RapidSpellWInline, you will probably find that the error underlines are not shown when the user clicks the spell check button.  The reason is that the ‘z-index’ of the modal popup is higher than that of the underlines.  This means that the underlines are hidden beneath the popup.

To solve that, add this style sheet snippet to your page.

    <style type="text/css">

    .oldBrowserBox{z-index:99999 !important;}

    #RS_CM_DIV {Z-INDEX:99999 !important;}

    </style>