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Microsoft Internet Security and Acceleration (ISA) Server 2006 may incorrectly return an "HTTP 304 Not Modified" response to a proxy client. For example, ISA Server 2006 may return an "HTTP 304 Not Modified" response to a proxy client when the content is changed or is expired.
This problem can occur if the client browser sends a request that includes the "If-Modified-Since" header field.
To resolve this problem, obtain and install the hotfix that is described in the following article in the Microsoft Knowledge Base:
951510 Description of the ISA Server 2006 hotfix package: April 9, 2008
Microsoft has confirmed that this is a bug in the Microsoft products that are listed in the "Applies to" section.