Content mismatch errors is when you have a web page on HTTPS that has some elements of content on the page that are not secure, such as images not over HTTPS, or social media plugins or whatever. It is actually a very common issue with HTTPS web pages.
In any event, we covered in the past that mismatch errors get the ranking boost, even if they are not fully HTTPS secure. But that may change in the future says John Mueller from Google.
I imagine at some point, we will say you will have to serve really clean HTTPS for us to show that in the search results. I don’t know. I don’t think that there will be any kind of penalty around that. Just that we may say, we won’t count this as HTTPS, so it won’t get that small HTTPS ranking boost.
Now, if you watch how he says it, you will see he is not 100% confident, but he wouldn't say this if Google wasn't thinking it.
Here is the video at the start point:
So get working on your HTTPS site and make sure it is not serving content mismatch errors. Not that you will see a penalty but you might not get that really really tiny ranking boost.
Forum discussion at Google+.