A few people have mentioned the problem of youtube bars breaking from old formats. As far as we can tell, on these forums a height value of 24 or lower will bug out the flash box, and it'll look like this:

Code:

[flash(400,23)]http://www.youtube.com/v/6hPU9Ru1ftw[/flash]

All you need to do is keep your themesong bars, or play buttons, at 25 or over. And then it'll look like this:

Code:

[flash(400,25)]http://www.youtube.com/v/6hPU9Ru1ftw[/flash]

If 25 height doesn't work, then god damn, you might need to use 28. Zaku has this.

Code:

[flash(400,28)]http://www.youtube.com/v/6hPU9Ru1ftw[/flash]

Hope this serves as a handy clarification.
- PSA brought to you by Krug