Recently the Large Hadron Collider was damaged by a mechanical failure. This requires the collider to be warmed up, repaired, and then cooled down again, so we're looking at a two-month delay.
Inevitably, many commenters said, "Anthropic principle! If the LHC had worked, it would have produced a black hole or strangelet or vacuum failure, and we wouldn't be here!"
This remark may be somewhat premature, since I don't think we're yet at the point in time when the LHC would have started producing collisions if not for this malfunction. However, a few weeks(?) from now, the "Anthropic!" hypothesis will start to make sense, assuming it can make sense at all. (Does this mean we can foresee executing a future probability update, but can't go ahead and update now?)
As you know, I don't spend much time worrying about the Large Hadron Collider when I've got much larger existential-risk-fish to fry. However, there's an exercise in probability theory (which I first picked up from E.T. Jaynes) along the lines of, "How many times does a coin have to come up heads before you believe the coin is fixed?" This tells you how low your prior probability is for the hypothesis. If a coin comes up heads only twice, that's definitely not a good reason to believe it's fixed, unless you already suspected from the beginning. But if it comes up heads 100 times, it's taking you too long to notice.
So - taking into account the previous cancellation of the Superconducting Supercollider (SSC) - how many times does the LHC have to fail before you'll start considering an anthropic explanation? 10? 20? 50?
After observing empirically that the LHC had failed 100 times in a row, would you endorse a policy of keeping the LHC powered up, but trying to fire it again only in the event of, say, nuclear terrorism or a global economic crash?
Originally I was going to say yes to the last question, but after thinking over why a failure of the LHC now (before it would destroy Earth) doesn't let me conclude anything by the anthropic principle, I'm going to say no.
Imagine a world in which CERN promises to fire the Large Hadron Collider one week after a major terrorist attack. Consider ten representative Everett branches. All those branches will be terrorist-free for the next few years except number 10, which is destined to suffer a major terrorist attack on January 1, 2009.
On December 31, 2008, Yvains 1 through 10 are perfectly happy, because they live in a world without terrorist attacks.
On January 2, 2009, Yvains 1 through 9 are perfectly happy, because they still live in worlds without terrorist attacks. Yvain 10 is terrified and distraught, both because he just barely escaped a terrorist attack the day before, and because he's going to die in a few days when they fire the LHC.
On January 8, 2009, CERN fires the LHC, killing everyone in Everett branch 10.
Yvains 1 through 9 aren't any better off than they would've been otherwise. Their universe was never destined to have a terrorist attack, and it still hasn't had a terrorist attack. Nothing has changed.
Yvain 10 is worse off than he would have been otherwise. If not for the LHC, he would be recovering from a terrorist attack, which is bad but not apocalyptically so. Now he's dead. There's no sense in which his spirit has been averaged out over Yvains 1 through 9. He's just plain dead. That can hardly be considered an improvement.
Since it doesn't help anyone and it does kill a large number of people, I'd advise CERN against using LHC-powered anthropic tricks to "prevent" terrorism.