11. There is a theory about Quantum Suicide
“Quantum Suicide/Quantum Immortality. The idea that we never really die in our perspective. Every time we encounter a situation where we may die, we continue on in a parallel universe where something happens that prevents our death. But we die in the original universe. In a sense, our consciousness lives on by transferring itself to a parallel universe where we continue to exist.” — BarbaraCripps
12. If the universe is under a false vacuum, we could be in trouble
“It has been hypothesized that our universe is under a false vacuum, meaning that zero energy isn’t actually zero energy, the whole universe could go lower, but for some reason it isn’t, like a marble on the second last step. If something nudges it off at any point in space, the laws of physics there completely change as the university rearranges itself fundamentally. This knocks the adjacent space down with it. The resulting chain reaction completely resets the universe, it happens at the speed of light, and we can’t do anything about it.” — Tethalamin
13. One machine could cause all of our deaths
This is somewhat similar to the strange matter mentioned elsewhere in the thread, but technological. A microscopic machine that is programmed to break down matter and reorganize it into copies of itself. A sufficiently advanced machine could replicate itself from common materials and multiply at an exponential rate. A single machine could begin to multiply exponentially, consuming all useful matter around it, until the entire crust of a planet is either converted into machines or piled into heaps of useless waste products.
This could happen accidentally. A scientist in the future could make a nanobot with a nasty software bug. A hostile alien race could use it to wipe out human civilization safely and quickly. Maybe this catastrophe has befallen other planets, and the nano machines are able to spread from planet to planet, being plucked from the upper atmosphere by solar wind, destroying everything wherever they make planetfall. Given enough time, goo could travel interstellar or even intergalactic distances.
And all it takes is one to doom every living thing on the planet.” — bunks_things