Back in the 2000's (age of J2ME featurephones and when 3G networks were only starting to flourish) there was this Spanish carrier that had a separate APN for sending MMS (with a proxy that only allowed access to the actual MMS server, that billed by sent message) and another one for ordinary data (which was, of course, expensive at the time).<p>After some fiddling, I found out that the filtering proxy was banning access to anything other than <a href="http://mms.provider.es*" rel="nofollow">http://mms.provider.es*</a>. Note how there was not a trailing slash.
You could access any domain like <a href="http://mms.provider.eswhatever.freedns.org" rel="nofollow">http://mms.provider.eswhatever.freedns.org</a> and it would happily proxy you to the outside internet. As the billing was done on the MMS server and not in the proxy, you could pretty much open any HTTP connection to any proxy that had a domain like that pointed to it.<p>Some deep browsing (too much free time) led to Filipino forums sharing hacked versions of Opera Mini and other popular apps that let you change the Opera proxy endpoint to other custom domain that then was pointed to Opera's own servers - probably because of similar separate tricks.<p>Oh, old times...