www.mumsnet.com/Talk/_chat/4372807-ds-glaringly-positive-lft-pcr-just-came-back-negative#prettyPhoto
I was on this page when it last happened and it was a video ad for Ellesse. I'm on an android using chrome with whatever the latest verison is.
I was scrolling and the advert scrolled with the screen so that it was covering the text of the thread making it unreadable.