Fake Pre-Roll: A video ad that auto-plays on a web page where the viewer has shown no intent to watch the ad or the content after the ad. Often seen playing in banners or small, syndicated players below the fold.
Learn more about "fake pre-roll" at the TubeMogul blog. TubeMogul uses multiple layers of technology to prevent "fake pre-roll" ads from running in its client campaigns.
While some brand marketers buy this ad unit intentionally, it is often used by ad networks to fulfill campaigns without the advertiser knowing about it. Or sometimes the publisher runs this ad unit without the network or advertiser knowing about it.
It's an industry-wide problem. We want to inform brand marketers about the issue, so they can demand more transparency into what type of pre-roll they are buying.