Here's a nifty trick which I hope can help you if you try to load something but you find that the page has been cached when you know it should not be cached.
Add a ? mark to the end of the URL. (For example, to load google.com, do http://www.google.com/?)
Why it works is left as an exercise for the reader. 
I originally learn this trick when the local CSG department in the school of computer science and engineering were experiencing cache poisoning problems by trojan horses that have infected Windows client boxes. Then, I subsequently forgot about it. 
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It depends on which proxy ser
It depends on which proxy server you're using. For squid the following line in /etc/squid.conf controls this behaviour:
hierarchy_stoplist cgi-bin ?
As for why appending a ? to the end of the URL does not change the actual URL loaded, well let's leave that topic for another day.