Prudhvi, could try to repro and triage this?

My guess would be that this is caused by one of the user's browser extensions. 

If you hit CTRL+Shift+J to open the Developer Tools console (or click the More Tools > Developer Tools menu item) it will show you the URLs of the unauthenticated resources that are attempting to load.

Can you try that jfz30302@, and paste the result into a comment on this bug?

It is an extension, here's what the error said: 
chrome-extension://gngocbkfmikdgphklgmmehbjjlfgdemm/inject/js/SBPageExtension.js:211 Mixed Content: The page at 'https://bugs.chromium.org/p/chromium/issues/detail?id=700361#c2' was loaded over HTTPS, but requested an insecure script 'http://www.sbx-media.com/extn/inject/js/ads_advertisement.js'. This request has been blocked; the content must be served over HTTPS.

What's strange is this only began happening after I updated to the newest Chrome Beta channel release. 

Thank you for providing more feedback. Adding requester "elawrence@chromium.org" to the cc list and removing "Needs-Feedback" label.

For more details visit https://www.chromium.org/issue-tracking/autotriage - Your friendly Sheriffbot

Interesting. A HTTP reference should always have resulted in this warning.

However, as noted in  Issue 700397 , distrust of StartCom certificates is getting broader. https://www.sbx-media.com uses a StartCom certificate.

It seems entirely possible that this extension tries to use HTTPS://www.sbx-media.com, sees that the request fails (due to the untrusted root) and then falls back to a HTTP URL (which is blocked as mixed content).

Ultimately this is a bug in the extension (which appears to be an ad-injector). 

I see. So should I disable the extension as it is falling back to a less secure HTTP URL, or is it okay to continue to use it? (Is it putting my browser's security at risk?)

Actually, no, this isn't related to the StartCom deprecation, and this error is observed in Chrome 56 as well as shown in the attached screenshot.

Mixed Content: The page at 'https://example/' was loaded over HTTPS, but requested an insecure script 'http://www.sbx-media.com/extn/inject/js/ads_advertisement.js'. This request has been blocked; the content must be served over HTTPS.
adBlockerCheck @ SBPageExtension.js:211

As this is a bug in the extension, there isn't anything Chrome can do about it.

As to the question in Comment #6: This extension appears to be injecting third-party content into all pages; this is inherently dangerous and I personally would not install any extension that did this. Having said that, the "Blocked Script" notification means that by default, at least, the script that is injected via HTTP into HTTPS pages is blocked. But that doesn't stop the extension author from updating their extension to use HTTPS injection and running their code on all of the pages you visit.

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I understand. I think I will uninstall the extension, now knowing this. I hadn't seen the message before updating, so I thought it might have been a bug in Chrome. Thanks for the information!