IF you set a page to be active in the future, it doesn't exist and attemts to visit the page will result in 404 error (page not found).
You can create a page (actually pages) in DotNetNuke and have them be the redirect target of various errors. This is configured in the web.config file. A very simple example is described at
http://www.dnnhero.com/Videos/tabid...-113.aspx. This is a .NET feature and is not exclusive to DotNetNuke. As the example indicates, you can create customized error pages that are different depending on the error. Google for ".NET Custom Errors" for more information.