When you retrieve a document with an extension that has a corresponding script map entry (for example, .asp, .stm, .idq ), the custom error messages configured to replace the default error messages are not used.

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In cases where the default error messages are displayed instead of the configured custom error messages, the following workaround exists:

1.	From the Microsoft Management Console (MMC) IIS Snapin, right-click the Web site and click Properties.
2.	On the Home Directory tab, click the "Configuration" button.
3.	Highlight the extension that fails to return the custom error message and click Edit.
4.	Select the Check That File Exists option.
5.	Click OK, and then click Apply.

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This behavior is by design.

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When a request is made to an IIS server for a registered file (such as an ASP page), IIS simply passes that request to the corresponding ISAPI extension. The ISAPI extension that is registered for that file type is responsible for the completion of the request, including the return of any errors to the user.

This allows the ISAPI extensions to return error messages that may relate more information than a standard error. When the "Check that file exists" option is enabled in the Add/Edit Application Extension Mapping dialog, this forces IIS to check and verify that the file exists, and the user has appropriate permissions prior to passing the request to the ISAPI extension. When this option is enabled and the file is missing, or the authenticated user has inappropriate permissions, IIS will return the appropriate custom error message.

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