I got this annoying pop-up while reading an article (from a reputable publication) on Flipboard. I had to force close the Flipboard app to dismiss the pop-up as there was no way to cancel or back out of it! Other than presumably tapping OK which I did not do.
An unanticipated pop-up is rude enough, but one that forces the user to proceed is downright coercive. Which makes me think this is either a dreadful implementation of a mobile advertising technique, or it is a scam.
