In the side bar on the right, I saw a cross-site "ad" for a Music question with an interesting title.
I clicked the ad, then was met by a browser alert with the warning
For security reasons, framing is not allowed; click OK to remove the frames
And the weird thing is, when I clicked the alert button, the ad was momentarily replaced with what appeared to be the top-left corner of the page I was on, then I was redirected to the front page of SO.
This is similar to this question, but has only happened the once, when I clicked on the ad. I have been unable to reproduce it by clicking on other ads.
I'm on Chrome "Version 71.0.3578.98 (Official Build) (64-bit)"
Our internet is awful right now (1.5mbps download!), and has been for a couple hours. Shaw's derping hard. Not sure if that has anything to do with it. I wouldn't think that it would, but I figured I'd mention it.