And... here's a list of sites that hardcoded to use HSTS. Theoretically, all these sites should give the problem and not pull up. mail.ru and amazon.com are included

My only guess is that this has something to do with Opera's VPN feature, as I've been using the VPN when this notification was received. This is of concern to say the least, as it brings to mind SSL man-in-the-middle exploits.

MY NEXT GUESS: This appears to be an issue with HSTS (HTTP Strict Transport Security).
From Wikipedia: "TTP Strict Transport Security (HSTS) is a web security policy mechanism which helps to protect websites against protocol downgrade attacks and cookie hijacking. It allows web servers to declare that web browsers (or other complying user agents) should only interact with it using secure HTTPS connections,[1] and never via the insecure HTTP protocol."

It also seems that some users may tweak Opera settings regarding HSTS by going to opera://net-internals/#hsts I suppose by adding some sort of exception as per: http://classically.me/blogs/how-clear-hsts-settings-major-browsers BUT...

I get forwarded to a web search when I try to go to that blog post about clearing HSTS for specific domains, so perhaps it is a bug or only perhaps the customization page is available in other Opera versions, like the one for Windows? EDIT: Now all of a sudden the link chrome://net-internals/#hsts IS working for me

Considering that the blog post was written in Feb 2014, it's strange that the issue would just be seen now, so I'm leaning towards bug.

My only guess is that this has something to do with Opera's VPN feature, as I've been using the VPN when this notification was received. This is of concern to say the least, as it brings to mind SSL man-in-the-middle exploits.