The [MEDIA=youtube]xCjMABqWdEM[/MEDIA][/code]
you should instead type
Code:
[PLAIN][MEDIA=youtube]xCjMABqWdEM[/MEDIA][/PLAIN]
As for the supposed problem with mentions, I think I've seen it in one of your birthday threads wherein you mentioned a large amount of people in one post. I assume you didn't retype all those mentions every single time but copied them from an older thread whenever you made a new one. Certain BBcodes morph after you submit a post (or an edit to a post). To pick an example that relates to the other question: the [MEDIA=youtube]xCjMABqWdEM[/MEDIA][/code]
into
Code:
[B][COLOR="#F37934"][MEDIA=youtube]xCjMABqWdEM[/MEDIA][/noparse]
provided the video is successfully embedded. You can see this by editing a post with an embed after you've submitted the post. Similarly, mention morphs the clickable username into a set of parsing BBcode once the post is submitted. Upon consecutive editing of said post, the mention (which has already become parsed BBcode)
may break if it somehow fails to parse properly.
This can be caused by simple factors such as having multiple mentions close to each other, lacking space between each mention, or possibly having too many mentions in one post. When this happens, the code will become what you just posted, which is HTML for the text formatting used for a mention (bolded, underlined, and links to the mentioned user's profile). As HTML is normally disabled for posting, the borked HTML code can't be properly reparsed.
Theoretically, the workaround is very simple: if you intend to copy and paste multiple mentions from one thread to another, it's better to have a .txt file or something nearby containing what you'd type to mention all those users. For example, if you want to mention all the admins, make sure what you copy and paste is a string like this
instead of
and ensure your post is properly formatted, mentions properly spaced and not excessively laid out.