Accidental World-eating Black Holes
Any particle physicist will tell you that the European Organization for Nuclear Research's (CERN's) Large Hadron Collider (LHC) exists to provide new insight into how matter behaves at the shortest distances and at the highest energy levels. A conspiracy theorist, however, will inevitably insist that it just might unleash a planet-munching mini black hole.
Let's just say that's at best a half-truth. Yes, the high-energy collisions inside the LHC could theoretically create a mini black hole, but physicists agree that such a fun-sized singularity would quickly lose mass and evaporate before it could so much as swallow a researcher's ink pen, much less Earth itself. Plus, high-energy cosmic rays more powerful than anything inside the LHC crash into our planet all the time, and we've yet to witness such a reaction.
How to prepare: The most important part of any emergency preparedness kit is a plan. Where will you seek shelter in the event of a storm? How will you exit the home in the event of a fire, and where should family members meet to regroup? Granted, none of this will be much use against black holes. Anything you read about singularity-proofing your basement, while entertaining, is probably a scam.
Not every cosmic doomsday theory starts out so small. Look out, because here comes Planet X.