You have sometimes this error while developping a SharePoint application :
Server Error in ‘/’ Application.
The resource cannot be found.
Description: HTTP 404. The resource you are looking for (or one of its dependencies) could have been removed, had its name changed, or is temporarily unavailable. Please review the following URL and make sure that it is spelled correctly.
Even if you have activated the possibility to see errors and trace in SharePoint (Web application web.config –> callstack=”true”, customerror=”off”, debug=”true”) you won’t have a precise indication of what ressource is missing.
So just right click your page and choose “view source”.