bad xml: externalId, level = 6

First off I have to say I’m not html literate at all but I have a major problem I desperately need help with. I embeded video code from a website today and ever since I keep getting this annoying “bad xml” when I’m trying to add content to my blog. I took the video coding off my blog but I’m still getting this drop down box that won’t go away. Can anyone please tell me what I can do to get rid of this?