Just wondering if theres anything about known proxy lists or sites that require their own authentication so as to blacklist them...reason being that I was testing some that I thought were ok but unknowingly they were from private proxy servers.

I'm a newbie so forgive my ignorance too ops:

Generally speaking if the Proxy is asking for Authentication then it wasnt tested properly in the first place. A proper test should eliminate all such Proxies first time.

Be more specific if you can.
When i scan for proxies and find servers with open ports ( i test to see if they are a proxy).

Most of them are bad (Not found, Forbidden, Authentication Required Ect)
and i get a few (Connection Refused, Connection Timeout, and a few working proxies ).

If you scan a certain way, and test proxies you scanned, you will come across a lot of these, but you shouldn't find these on forums/public lists.
The bad ones should be (Not Found, Forbidden, Connection Refused, Connection Timeout). You will rarely see the Athentication Required label at all.

So to answer your question, you proably come across a list that was scanned, but not tested. :?