If one beautiful morning your SharePoint designers complain that they now get "The server sent a response which SharePoint Designer could not parse…" error when they try to edit SharePoint page or page layout, and/or your SharePoint developers all of the sudden are unable to access any of the SharePoint services under _vti_bin folder (like _vti_bin/lists.asmx), then check your web.config file. The tags in <httpHandlers> section in web.config file are probably out of order.
<remove verb="*" path="*.asmx" /> tag needs to be posted before any of the <add > tags in <httpHandlers> section in web.config file, otherwise the SharePoint web services path configuration breaks.
In our case this issue was caused by the web part install that incorrectly modified web.config file to configure AJAX in SharePoint. I guess the lesson is always be cautious of the automatic web part installers J