When the LHC was starting up, there were some worries that a collision of two particles could form a bubble of "true vacuum", as opposed to the "metastable vacuum" that the universe is in today. This bubble would then expand outward at the speed of light, annihilating any matter that stood in its way.

 

Studies later found that cosmic ray collisions, both past and present, took place at energies much higher than those that human-made collisions are capable of, which meant that the LHC is safe.

 

However, sooner or later, humans will be able to create collisions that are as powerful as, and eventually more powerful than, naturally occurring collisions. Could one of those collisions set off what's known as the vacuum metastability disaster? In other words, could it be that there's a certain energy barrier that naturally-occurring collisions can't overcome but that human-made collisions could overcome? After all, it's not that far-fetched to think that humanity can create collisions that are more energetic than natural collisions. There are several things we can do better than nature.

If this were true, scientists building colliders would not be the only thing you would have to worry about. Any barrier can be tunnelled through with a non-zero probability. So the universe would have a very small but non-zero chance of just disintegrating from one moment to the next.