Have you ever received one of the errors below while browsing a web site?
This could be caused due a response from a web server that begins with a space or tab character in the HTTP Header. If you have ISA Server 2006 SP1 the fix for that is already built in, however you still need to create the registry key described in KB935693. This KB has an example of the HTTP header that was captured using Netmon and how it looks like.
Note: This KB was also reviewed for TMG MBE and also applies to it.