I have worked in video production in the past, I can not think of any graphics software or tool that accidentally superimposes an X on anything. This would have to to be an X - probably generated by some text tool that a switcher could turn on and off.
They probably thought no-one would notice.
The only time I've seen an X superimposed on the screen is when a transition has not been rendered yet. Has anyone contacted Chyron, or whomever it is that makes CNN's character generators, to find out if they have any pre-programmed screens that look like that?