As someone who works in a television "control booth" (local, not network) I can understand CNN's lame but plausible explanation. The full explanation is far too long and technical to try to explain, and most people wouldn't understand it anyway (certainly not the majority of CNN's viewers, and likely few members of CNN's management team would even get it). "Computer bug" is a dummed-down way to put it, but the comparison is accurate enough for most people to grasp.
Most televised sporting events use a similar process when effecting from live game action to an instant replay and back, although from what I've seen, most production trucks are smart enough to use the word "cued" (or something similar) instead of a big fugly "X" at the start of their video clips. Plus, I've not seen the "cued" message so big and obvious. Usually it is smaller and unobtrusive so as to be less noticeable in the accidental event that it makes it to air (which I've also seen happen). This frame is never intended to be aired; it is simply a reference point to indicate that the video clip is ready to roll. (Most facilities run these clips off of hard drives, although videotape can still be used.) The "switcher" rolls the clip, then one or two frames into the clip, (after the reference frame is gone) the switcher "switches on" the TV signal for that clip, adding it to whatever other video source you are seeing. At the end of the clip, the switcher turns off the signal from that clip, so it can then be recued for later use, without the viewer seeing the clip recue.
Any of a number of things could have happened at CNN to cause the switcher to turn on the clip's signal without actually playing the clip, thus causing the "X" to have been visible when it shouldn't have been. I can't help but chuckle at those who believe the mistake was intentional, although I must admit that if somebody wanted to pull something like this off, it would not be difficult to override the system and do it. It seems far more likely, though, that it was in fact a technical glitch.
But... if CNN had used the word "Cued" instead of a big X, I doubt any of us would have given the mistake a second thought -- it would have been an obvious technical problem -- still embarrassing, but more easily forgivable. The fact that it was a big X and the fact that it appeared over the Vice President makes a simple oopsie that much bigger. I'm not saying that anybody is making this bigger than it is, though. Even with as much heat as CNN is taking, I personally don't think it's enough. Technical issues happen, but this didn't need to be as ugly as it was. Using a full-screen X is stupid and unnecessary. CNN deserves every bit of embarrassment this error causes it.
I believe audiences are much more forgiving when a mistake is obviously just a mistake. But when the mistake comes off looking as if it could have been intentional, there will always be those who assume the worst first. That's human nature. And given CNN's well-earned reputation for being easy on libs and overly critical of conservatives, there's all the more reason for concern, and all the more reason for CNN to make extra efforts to avoid this kind of thing from happening in the first place (assuming they care what we think of them).
I doubt we'll see heads roll (employees canned) over this mistake. But imagine if the big X would have appeared over, say, Hillary Clinton. You can bet both buttcheeks that CNN would have pink-slipped somebody, although probably somebody completely uninvolved with the problem.
I think somebody at CNN should be canned over this. Basic TV 101, rule 1: anything you put on a tape (or a hard drive) could go on the air at any time. If you don't want it to air, don't make it available! I certainly don't think anybody could have foreseen the big X appearing over the Vice President of the United States, but, whoever created the video clip should have had enough sense to think that the X could accidentally air at some point. The same problem would have been embarrassing if it had happened over a CNN anchor or reporter, albeit not as embarrassing as what actually ended up happening. In my opinion, CNN should immediately rebuild their video clips to use "cued" (or at least something less potentially embarrassing) as their reference frame, and making it smaller wouldn't hurt. But when was the last time CNN paid any attention to anything we suggest?
If I wanted to do something like this, I would have to be aware that the management might not appreciate it. So, I would take pains to create the event in such a way that it would be easy to explain it away as a "technical glitch" if I were called on the carpet. Hence, adding a stock message like "Transition begins after 5 frames of black" would be a perfect cover. I would know that the audience would only see the "X" and never be aware of the message below it. I would also know that it would only be a matter of hours before the mystery message would be deciphered and become known to the management. A perfect preemptive CYA, IMHO.