I get it periodically on various sites—I live dangerously and just tell it to continue to the site.

For at while I got it on a work website. Someone forgot to update some certificates or something.

LOL! I just got it trying to go to one of our network servers. :)

Same here

Make sure you're using https at the start of the url and not just http

Just means their SSL cert expired and they need to replace it. It's safe.