I'm completely serious, I haven't seen an add on youtube ever!
You don't have to believe me. :-) I enjoy not having to watch them.
I must say this is an interesting response.
I do have a very extensive list of blocked names in my hosts file.
An small annotation link to a different song appeared for a couple of seconds, at the bottom left. Those annotations can be disabled in the ui. Of course that as any other annotation, didn't pause the video.
Apart from that there were no adds.
( choose a better song next time please :-) )
(I didn't look closely at it -- was in a "popular videos" cluster and VEVO stuff will always have ads)
I'm sure that careful readers( not that many apparently :-) ) will spot the cause.