Mixed Content: The page at 'https://stackexchange.com/' was loaded over HTTPS, but requested an insecure image 'http://www.gravatar.com/avatar/...'. This content should also be served over HTTPS.
I only noticed because I've enabled HTTP Strict Transport Security in chrome for SO and SE.
The issue seems to be the randomly generated gravatars in the "Top Network Askers" sidebar. Notice that the custom ones are being served properly over https.