I'm sorry if this aggravates anyone, but I have been fixing peoples embedded YouTube videos a little too often lately.
Here it is, step by step, folks!
1. Head over to YouTube and find the video you want.
2. Copy the highlited portion of the URL. (See Picture)
3. Start posting a reply, just like you normally would.
4. Click the little YouTube button. (See Picture)
5. This will put the YouTube tags up for you.
6. Paste what you copied in between them. (See Picture)
7. Click "Preview Post" to make sure it works. If it doesn't, repeat previous steps.
8. Click "Submit Reply"
I hope this helps, guys!