People dismiss that as astronomically unlikely, but it must have happened many times somewhere in the universe. The average person doesn't win the lottery once in their life, but someone does.
I believe we're beginning to appreciate that just like there are more small objects in the solar system than large ones, and there are more small stars than large ones, there are significantly more solitary planets than stars or even brown dwarfs, and they would be almost as hard to see coming as a black hole until they got close.
There are many disasters that at least microbes could recover from, but anything the size of Mars that wandered in would, I presume liquefy and sterilize the entire planet. It would be so unexotic and yet humans would be completely impotent to do anything.