On mobile (not app, mobile site using iPhone 5c) I keep getting the advert on the attached. It's designed to look as if it is part of the site in the hope you'll click to read messages. It tried to do weird stuff the first time I saw and accidentally clicked.
I've had this too. It is really invasive, my toddler grabbed my phone he needed more tayo apparently and hit the ad by accident and once it's on the screen the only way I could get rid of the pop up was to go into my settings and clear the whole browser history and cache that way. It has a couple of different skins.
I realise the ads probably aren't controlled by mumsnet, but the service that provides the ads, but that particular advert is sneaky as hell.