You are so correct ... I too am a former newscast director and this was no "computer glitch". It was caused by a human, not a computer. Deliberate or not, someone in the control room did this by screwing around!
Dittos from a former Air Force Armed Forces Radio & Televison broadcaster. Iv'e directed and T.D.'d many live newscasts and other productions and fully concur that this was the control room guys screwing around, putting the "X" over Cheney's face in preview bank and it got on the air by mistake. The "X" had to have been created on purpose and available as a video source on the switcher.