FIREWALLS OR PROXY SERVERS can sometimes cause problems if they are 
misconfigured. 

Personal firewalls (such as BlackICE, AtGuard, Norton's Internet Security 
Personal Firewall, etc) must not be configured to block the HTTP_REFERER.
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This may be referred to in several different ways, but it's always part of 
the HTTP Header. Sometimes you are told that this is for your privacy, but 
the HTTP_REFERER is only sent, indicating what link you followed to get
to the next picture page. It sends no personal information.