lol, and people wonder why they shouldn't use those.  Nothing free is without a cost, usually way more than what it's worth.

Really good video and verry funny. You can never be sure to be safe.

The last time i used proxies was years ago and all of them came from automatic scans rather than premade lists. Most of those were wrongly configured servers which were relatively secure. On the other hand even then you had to make sure not to scan some IP ranges as they include honeypots and government servers you would better not touch.
 

You can never be sure that you will avoid honeypots but there are huge lists of IP addresses known to belong to government agencies and other adresses you want to avoid. Generally spoken you should avoid scanning manually all together to not get in trouble.

https://www.iblocklist.com/lists.php

I think your best bet to have some anonymity on the web is to use TOR. Or if you are not doing anything illegal don't use proxies at all. Like you can see in the video they can undermine encrypted connections like https which is actually there for increased privacy. The average user does not need any proxy for everyday browsing.

If spying agencies or malware is of a concern to you you could always use those blocklists as addition to your hosts file to block them alltogether. You can never be sure a proxy is safe giving you a false sense of security/privacy.

Speaking of MITM...

http://www.techworm.net/2015/02/superfish-visualdiscover-lenovo-pcs-come-with-pre-installed-adware-and-mitm-proxy.html