If you use Chrome, the chances are that you've seen the above image. It's shown, for example, when a page is in the Google SafeBrowsing blacklist or when its SSL certificate has expired. Normally, you can bypass this warning by clicking "Advanced" then "Proceed to whatever.com (unsafe)" but, if the website is using a special technology (called HSTS), the above method will not work, and you'll be presented with this message instead:
I learned recently an easy trick to bypass this. Just type
thisisunsafe, and Chrome will let you into the website.
While this protection is in place for a reason and it isn't a good idea to use this trick every time you get into a warning screen, this may help you access some sites you wouldn't be able otherwise (old sites whose certificate has expired or even your own site during manteniance).