Contrary to what the sibling posters are saying, they could do this if they wanted to. Most modern video formats can be sliced and spliced at certain frames wi

Contrary to what the sibling posters are saying, they could do this if they wanted to. Most modern video formats can be sliced and spliced at certain frames without needing to recompress or anything like that. If you operate all your own cache servers (which youtube do, AFAIK) you could splice a different ad into every stream, in real time. Youtube would have to be careful, as plenty of users are used to being able to refer to timestamps in the video. Right now people are used to telling someone the interesting bit is 2735 seconds into that video lecture, and that's a time without ads counted.