On this computer, I was logged into the network on Meta and SO, but not yet on SU. I wanted to cast an upvote, so I clicked "Log in", which led me to the regular login page with this url:
I clicked facebook login (like I always do), which led to this page:
It told me that my request was "suspicious" and "could not be completed". But, in spite of that message , my request apparently was processed. You can see from the freehand circle in the screenshot that it did in fact log me in (and when I went back to the original question and refreshed the page, I was now logged in).
As far as I can tell, I did nothing out of the ordinary here. Not a big deal at all, but definitely seems like a bug to me.