Apparently at least some of the cometary debris fields the Earth passes through contain several hundred meter sized chunks of the old comet. Collision with one of them would flatten a continent. Whole comets colliding with Earth might destroy almost all life on Earth.
But neither of those is the most likely way for humanity to end our hour upon the stage. We are still our own enemy number one. Sixty some years into the thermonuclear age, we still haven't blown ourselves up, but the hazards are growing again, as more nations get the bomb and missiles to deliver it. The world's most dangerous nuclear power is now run by probably its stupidest leader ever, and another major nuclear power is aggressively expanding.