I visited a website in Internet Explorer and a little yellow bar appeared at the bottom of the browser warning me that the HTTPS site was trying to load HTTP content (AKA mixed content). I was surprised to find out that Chrome's warning is a lot more subtle:
A very subtle shield is displayed in the top right corner to indicate that Chrome has protected you from the HTTP content, in my case a single CSS file. This is something that developers, or even penetration testers, could easily miss.