I get that and I use Ad Blocker Plus. It does not seem to always happen. If it is part of a non YouTube site it sometimes seems to come up but not with the YT controls for full screen, etc.? Or maybe I am just remembering it wrong. To disable Ad Blocker Plus, go to the tiny
red stop sign in the upper right corner of your screen and click on it. It has the options to turn it off for specific sites. After watching, you can turn it back on or not. Your particular ad blocker may differ in how to do it.