You know what's truly mind-boggling about this science is that each of these near-misses... every time an asteroid passes relatively close to the earth, it affects our orbit just a bit, and objects passing by other planets affects their orbits, and so on... so it's not just knowing where they are in the sky but making the taking all the gravitational variables into play when estimating a closest possible approach.
Also important to keep in mind that "close" is anywhere between the moon and the earth (distance wise), and that's about 239,000 miles ... and the "big objects" we're talking about are all well under a kilometer in size for the most part. So even "close" isn't close, technically speaking but it's kind of freaky to look over the tables, run the java simulations over a period of a few years, etc.
My official guess: all the Hollywood scenarios would never happen. If officials really feared a big-time impact, and they thought it was species-threatening, they wouldn't tell us IMO unless they had a great plan in place for deflecting it. IOW, the one that kills us, we'll never see coming (from our perspective). Governments will decide not to cause a panic / mass hysteria, try to squirrel away some important people and human knowledge in a cave somewhere and hope for the best. Deep Impact / Armageddon scenarios will never happen.
...into the light of a dark black night.