In this article we will see how to set a custom error 404 (page not found) error page for SharePoint.

1. Log on the sharepoint server and access the language folder in layout of IIS (% systemdrive%\Program Files\Common Files\Microsoft Shared\Web Server Extensions\12\TEMPLATE\LAYOUTS\LangID)
   Note: In this path, LangID represents the actual Locale ID of the language that you use. For example, 1033 is the language ID for U.S. English.
2. Rename the error.html file to error.old.
3. Create the custom HTML file (pagenotfound.html) and copy it to above folder.
4. Create a console
5. application / PowerShell script for assigning the new custom error page for your web application and run / execute it in SharePoint server

C# code for console application

Microsoft.SharePoint.Administration.SPWebApplication webapp =
Microsoft.SharePoint.Administration.SPWebApplication.Lookup(new Uri("http://"));
webapp.FileNotFoundPage = "pagenotfound.html";
webapp.Update();

PowerShell code

$webapp = [Microsoft.SharePoint.Administration.SPWebApplication]::Lookup("http://")
$webapp.FileNotFoundPage = "pagenotfound.html"
$webapp.Update()

Note: By default, in Internet Explorer, the Show friendly HTTP error messages setting is turned on and the custom error page may not appear. In this case turn off this setting and try.