Case 1: AllowDeletion

There might be a case where you have custom document libraries in your SharePoint-solution bound to a customer specific custom content type. It might be confusing for the customer to have the default libraries, which come when the Office SharePoint Server Publishing feature has been activated.

“Some user will at some point inevitably put…

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…