I have also seen cases when people tried to post nonstandard youtube urls that are legitimate but won't work with forum code.
For example, if you go to https://www.youtube.com/watch?v=rW6M8D41ZWU
you can see the Yatta video, but you can't embed it here unless you manually change the first part from "https" to "http". There's also (or at least there used to be) a youtube mirror that looked something like "youtu.be" and I don't think those worked either.
"It's so easy, happy go lucky..."