Well, it was bound to happen, the question was less when so much as it was how, and it’s an interesting addition I’d say uses inventive techniques.
Case in point- Ever Broke an Aluminum Bat (which actually doesn’t show any breaking of an Aluminum Bat, sad to say) loads with a yellow divider icon on the time lapse bar: 
As soon as your time hits that bar, an add will slide out from the bottom window ala slideshow pro style that has an x in the top right of the ad to close: 
If you close the ad, it will leave an icon to expand the ad again: 
Upon video completion, you’ll see options to view other YouTube videos of similar flavor, and the ad will return, without the option to close it this time: 
Goodbye TV ads, hello YouTube and the next generation of marketing.