In order to pull that little trick, they had to key an animated "X" in every fifteen frames or so.
It does look like operator error. Somewhere along the line someone probably puched up the CG (Chyron, Character Gen). -keyed it in - and the text had a flashing X in it. I bet it was most likely that someone was playing around in a control room to cause this.
If it was an error, how come errors like this don't happen occasionally at other times?
It could have been. The thing that bothers me, though, is the fact that this wasn't a normal video feed. The only way it could have been created was to be animated frame-by-frame. The only way it could be seen was by someone who could step through each frame of video to see what was there.
Unfortunately it appears no one has ever heard of any similar occurance of "fooling around" in thousands of hours of TV for thousands of viewers.
Although your explanation is plausible, it is not probable. This X is consistent with other behaviour in the media, and given that in millions of hours of viewing no one can think of a similar event - it appears intentional.
I would expect the only unintentional part of this story is that the people responsible will be caught.