Apparently there has been 1 episode of the MythBusters that has yet to air in America, but HAS aired in Austarlia, using their narrator.. It was supposedly set to air as last season's finale (not the Explosions A-Z Episode), yet it never did, though Previews for this episode WERE SHOWN.
This episode Involved Myths involving Boarding a Plane, and Making a bullet out of a tooth . Until a US air date is confirmed for this episode, it wont be added to the guide (along with their results)
Those who really want to see this episode, it is very easy to find online and watch that entire episode with the Australian Narrator. OR you can just wait til it DOES air here (If it ever does)