In the reply box of a thread, click on the Media button and note the list of sites you can embed (Dailymotion, Facebook, Liveleak, Metacafe, Vimeo, YouTube)
Now add or remove a media site in the AdminCP, or just change the name the title of an existing BB Code Media Site. Reload a page on your test forum and hit the media button. Note the changes didn't take effect.
The ajax result from 'editor/dialog&style=x&dialog=media' gets cached, so if you change a media site, your visitors won't get the updated description until their respective caches expire and it fetches it fresh.
The changed media code will work, but users may be thrown off the by the stale description.