I'm not sure any proxy script is. It's possible to carefully craft a page so that the script doesn't see a resource and doesn't rewrite the address to point to the proxy, hence fetching it with your real IP. I use it only where this risk is acceptable.
A small proof of concept:
http://www.frostjedi.com/terra/scripts/ip_unmasker.php
"I just remembered something that happened a long time ago."
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