* The TransparentProxy resolves the destination host again which may be different to what your machine resolves it to, for instance if you use an AlternativeRoot[1], or have a /etc/hosts (hosts(5)) entry for the machine the TransparentProxy will either fail to find the [DNS] for it, or even worse, find it's own one (which may be an old cached incorrect version). 

 * Your [ISP] uses software that is not clever enough. For example, [Xtra]'s proxy (used to? still?) didn't look at the "Accept-Language" header. For example, if I send a request for the home page of www.debian.org with Accept-Language set to French, and you later send a request for the same page (but your browser sends a different language for Accept-Language) then you might get the French version if the proxy decides that the page is not yet expired. ''That would explain why I kept getting debian pages in french! Why couldn't you just read them in plain english like everyone else? <grin> -- PerryLorier'' 

  

 [1]: Not a good idea at the best of time anyway.