I've looked this up on the internet and run virus scans, but it won't go away:
I haven't seen this pop up on any computer other than my own, which I'm pretty sure is a major red flag.
It's also strangely specific. If the video is embedded on another sit, the ad doesn't pop up. But if I watch it on Youtube, it does.
Another odd detail. When a Youtube video finishes, inside the screen a bunch of suggestions pop up. When you click one of the suggestions, it reloads the page to the next video. But now, for whatever reason, it plays the new video just right inside the old video page.
I'm flummoxed.