I've been watching T.V, since 1949. In all of that time, I have NEVER seen anything at all like a big black X pop up over someone's face, like this. And no, I am NOT buying the stupid "glitch" excuse.
Not over Earnie Kovaks, not over Ed Sullivan, not over David Galloway, not over Boston Blackie or Howdy Doody. Only over Republican politician Dick Cheney.
Standard NTSC video is 30 frames per second. To pull this off, an animation had to be created, striped to tape, and overlayed or keyed over the Cheney feed.
It's possible it was a mistake. It isn't likely it was a mistake.