If you get the following verbose error when trying to open a site in Sharepoint Designer 2007:
“soap:ServerServer was unable to process request. ---> A Web Part or Web Form Control on this Web Part Page cannot be displayed or imported because it is not registered as safe on this site. You may not be able to open this page in an HTML editor that is compatible with Microsoft Windows SharePoint Services, such as Microsoft Office SharePoint Designer. To fix this page, contact the site administrator to have the Web Part or Web Form Control configured as safe. You can also remove the Web Part or Web Form Control from the page by using the Web Parts Maintenance Page. If you have the necessary permissions, you can use this page to disable Web Parts temporarily or remove personal settings. For more information, contact your site administrator.”
Under the account of a user that has rights (usually the admin account), go to the URL and add “?contents=1” to it. This will list all web parts that have been added at any particular point. If you have a web part that you added at one point and deleted it through the URL, it still ‘exists’ and so if it wasn’t a safe part, or is no longer listed in the web.config as a safe part, it will be listed as an ‘Error’ web part. Select it/them and delete them, and then you should be able to open the page in Sharepoint Designer as expected.