In playing with custom errors i found even though i set the <customerrors> and debug section in my web.config like those below

<compilation debug="false"/>

<customErrors defaultRedirect="GenericErrorPage.htm">
        <error statusCode="403" redirect="NoAccess.htm"/>
        <error statusCode="404" redirect="~/myerror.htm"/>
    </customErrors>

I found these settings were still getting ignored until i set the retail flag in the Machine.config.

    <system.web>
    <deployment retail="true"/>

</system.web>

…now i don’t know if this is a change with the .NET framework in Windows Server 2008 -or i had forgotten i had done this in the past but it is working this way now.