Big one is if its a 'known' proxy they will blacklist.
Its complete bollocks that you ever have a private proxy UNLESS you are the first user of that IP address and the only way you can guarantee that is to get it straight from the likes of ARIN etc. However, as IPv4 is in very short supply they are calling back unused ranges so good luck with that.
In my opinion anyone saying you have a private proxy is either trying to sell you the proxy or lying. It may be private to you at that moment but you don't know what its been used for before by the last guy to use it - they may of spammed the ass out of that proxy, got it blacklisted etc and you still think you have a private proxy.
Anyway, I have gone off track due to my dislike of so say private proxies being pushed to people.
There are lots of ways of identifying proxy use especially if its off a VPS or something like that - reverse dns. Couple that with crap abused proxies that leak your real IP, like someone said browsers and webrtc, caching on your machine, cookies, local storage, browser type, screen res, browser useragent etc.......there are many ways you can get caught out.
Messing with Google just means they have a lot of ways of identifying you - they rule the web afterall
