Pages within the help center have many non-HTTPS links on them.
Many of the ones to questions seem to have been updated, but there are non-HTTPS links to the Stack Overflow blog on Don't Ask, links to Area 51 and the main domain of stackexchange.com on On-Topic, a link to a Main Meta on Closed Questions, a link to the Stack Overflow blog on Duplicates and presumably more of the help.
These were tested on the Meta help center, too, and the links that are on both I confirmed to be non-HTTPS.
