- Wikimedia Labs' HTTPS certificate
https://en.wikipedia.org/wiki/en:HTTPS expired for most of Tuesday 15 September. To reach Wikimedia Labs and Tool Labs securely you had to ignore your browser's security error message. There was no risk to data security during this time. [9] https://phabricator.wikimedia.org/T112608
Well, it would have been an ideal time to launch a MitM attack. It is very very unlikely this happened (IMO), but certificate errors are not without any security repercussions at all.
-- -bawolff