Back in the 1970s when CERN started doing pi meson production, a paper was published in a Russian journal that suggested the experiments might destroy the universe. I heard about this at a talk in Edinburgh title The vacuum as seen from Moscow.
You see pi mesons are produced by colliding heavy nuclei at relativistic speeds. The paper claimed that this had probably not happened before. When the ambient energy density of the universe was high enough to accelerate particles to ultrarelativisitc speeds there were no heavy particles, this was after the big bang during nucleosynthesis.
The heavy elements didn't get produced until later when stars had formed. By this time there was not enough energy lying around to speed the particles up.
The Russians claimed that if some models for the vacuum were correct then we lived in a universe which had a false vacuum with a lower energy vacuum beneath it. The acceleration and collions of the heavy particles could cause a tunnelling event between the two vacua. Now the true vacuum would be in a lower energy state then our false vacuum and the initial tunnelling event would continue as our universe fell through to its lowest possible energy state. The breach would expand from its point of inception at the speed of light.
People stopped the experiments for a while and thought about this. Then someone realised that no, actually there are places in the universe which have heavy elements and much higher energies than can be produced in experiments on earth. These are the cores of galaxies where AGN are formed.
If the Russian argument could hold true then it would probably already have happened at the heart of some AGN.
Since we were still here and hadn't seen anything like a wall of false vacuum approacing us from any direction in the universe (though as bdonlan pointed out how do you see something moving at the speed of light until it reaches you!, hey I was just reporting ;) ) then it was likely that the experiments could go ahead. Indeed they did and quite a few Nobel prizes were awarded on the way.
In response to lawnjart, the toys we have to play with are feeble in comparison to the energies available to astrophysical objects. I have every confidence in our ability to destroy ourselves, and it is my belief in our insignificance that gives me confidence that the universe has nothing to worry about.