The main reasons the Chelyabinsk meteorite was missed were 1. It came towards Earth in the direction of the Sun so it was not visible early on and 2. It isn't as large as most NEO's tracked at the time. Chelyabinsk was an asteroid about 20m wide, the asteroid that passed Earth the other day was 1 km. Good news is, our tracking has gotten a lot better in the last decade! There are around 18,000 known NEOs, with about 40 new ones discovered per week. There is always that rare possibility we miss one, but generally speaking, if you hear about one that might hit from the media before any governments comment on it, its probably perfectly safe! ;)