Ok, so this topic is a little old, but I felt I needed to chime in a little to add some context. First, I was in a band that opened up for David Lee Roth one time, and didn't stick around for his show. Not that I dislike him, but it's just that I don't care that much. So I come here to neither praise him nor bury him. :) First the poster who says that there is a keyboard player offstage is most probably wrong. Even in bands that do not have a keyboardist as part of the "band" they are almost always onstage but not prominent. Plus, I played keyboards, and we're not all boring to watch:) Second, if it was a keyboard issue playing live, the fix would have been simple and immediate, by tuning the keyboard, but tuning a keyboard is a very deliberate thing to do, and it would have been noticed by anyone with a good ear that it was badly out of tune in the opening musical section. And the reality is, there would be no good reason to detune a keyboard to another key. You don't play that song thousands of times and not notice that it starts out out of key. Therefore, this had to be (unless everyone in the production of Van Halen is an idiot without an good ear) digital recorded playback, but played back at the wrong speed. This would have been a simple mistake, but no one probably knew why it was done or if it was done on purpose until it was too late.
Also, EVH was probably trying to tune his guitar to the keyboards, but this is also difficult if not impossible to do six strings on the fly, since it was such a massive change. After one string was tuned, tuning the second string would detune the first one, and so on. A bigger trainwreck would have ensued. I believe he was trying to transpose (which would have been easy it was a keyboard problem, as they are most often "tuned" by key, except for the old analog keyboards).
To sum it up, this was not a keyboard problem, not a guitar problem, but a playback problem. The the poster who wonders about the Gwen Stefani situation, often the case is that there is a backup singer with a similar voice singing the lead parts to cover when the singer is out of breath. It is also popular to sing along with a prerecorded synced track for the same purpose.
Personally, I would have felt the thing to do was mute the keyboard track and ad lib. That's what my band would do in the case of an electronic glitch (we had many over the years) and it was always easier to recover from. But that also requires the freedom of the soundman to do what he thinks is right, and I doubt the soundman is "part" of van Halen, and so this would not have been an option. The soundman could have also fixed the problem by running the keyboard track through a detuner and fixed it that way, but that would require the soundman to know what the problem was to begin with. In the end, it could have all been solved by flicking one little switch, from 48.1Khz to 44.1Khz. Amazing. KNOWING that's all it would take takes an experience like this one.
Sorry for the long post, but I hope this clears some things up.