I got this popup for the second time now. (This time by crypt SE)
This doesn't look like anything we would serve.
If you are browsing over http, try using https to see if that happens again. I suspect shenanigans by your ISP or someone between you and us.
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Sure! I'll try. Sadly it's not reproducible everytime so I can't say for sure if it works. – Limit Nov 11 '16 at 13:35