I worked on a web application that started by comparing whole IP addresses on each access, and we started to have quite a few reports of people behind proxy pools having a problem
Really? I figured this sort of thing would be a very fringe condition. I figured that most people weren't behind proxies at all, and that of the proxied people, most only had one. Then of those that had multiples, the auto-proxy configuration script would pick one of the pool at random, or round robin. The concept of sending different requests from the same browser through different proxies seems counter productive to efficient caching...
perl -l -e "eval pack('h*','072796e6470272f2c5f2c5166756279636b672');"