I've been in the video business for 20 years. I never worked in news, but I've worked in control rooms, and I can see how this could be an unintentional error . . . IF it only appeared a few times.
There should be lots of eagle eyes watching the Air monitor closely. It's inconceivable that nobody would have seen this as it happened, certainly by the third or fourth appearance, probably on the first blip.
Then the mad dash to find the source on the master switcher. That might take a few seconds, or even a minute or so, depending on the repeat frequency.
How long before the X flashes stopped appearing, and how frequent were they?
The director and producer have the monitors right in front of them. It shouldn't have taken them that long to notice something wasn't right.
You can view the video. It is linked from Drudge's page.