Developers, who use customized page layouts bound with custom content type, have probably realized that if your custom content type is not the default content type of Pages library, you will not get default values populated when you create a new page. One way to work around this is to create feature, which sets your customized content type to be…

If you are working with SharePoint 2010, see this article by Todd Carter:
An Expected Error Has Occurred
Mike has also written a nice article of the other options:
SharePoint 2010 Custom Error Messages for Public Facing Deployments

SharePoint doesn’t have an out-of-the-box support for custom error pages. Or at least it’s a little bit problematic to implement custom error pages…