Why aren't my custom error pages showing up in Internet Explorer?
This answer applies to Customer Portal page sets, Internet Explorer (IE)
Oracle Service Cloud
When custom error pages display as expected in all browsers except for Internet Explorer (IE), it is often due to the minimum byte count restrictions for IE.
Custom error pages must be greater than 512 bytes in size. If they are not, IE will instead display it's own default error page.
For additional details on configuration of custom error pages, see Answer ID 5879: Custom Error Pages.