It seems like just a technical thing on the feed, but I'm not in the business.
I got very curious and annoyed my kids while I went through it frame by frame to try and read it.
When I finally got the right frame on the DVR, it read something like, "Yellow filter #201 then fade."
I think its definitely possible that the same sort of thing happened to CNN, but all they need to do is show some technical reason why it was there to be accidently flashed. Or fire the wiseass jerk who thought it would be funny to load that black X into the machine and flash it over the vice-president's face.