Ok I'm leaving this here for evidence so that the higher powers can try to fix it.
Though this has happened even before the spoiler tags, like if someone tries to add extra tags to youtube videos or something instead of just pasting the link.
Ok so the techies pretty much cant fix this. I did some testing and basically there are two different things regarding youtube videos that may happen to break the thread. This is a result of youtube videos being able to be embedded now.
So, #1 if someone quotes a spoiler youtube video as you saw, it will break the thread.
And #2 if someone tries to insert the IMG tags by mistake on a youtube video link, it will break the thread.
There may be more instances in which this occurs but I tested multiple different ways and these were the two that broke the thread.
So we need to avoid doing these two things, otherwise either the embedded youtube feature may have to be removed, or the spoiler tags may have to be removed.
If it happens on accident let a mod know so we can edit the post.