There are many, many asteroids and comets, and it is currently impossible to track all of them. So in short,yes, it is possible one could be on a path toward Earth. But, even though this is very likely in the next 500,000,000 years, it is very unlikely in the next 100 because these events are so extremely rare. Currently over 90% of the asteroids that are big enough to cause serious damage are already being tracked. Right now, NASA is working on a program to identify any of the ones that are missed, by combining data from telescopes all over the world. There actually was an article in the newspaper about it yesterday USA TODAY, October 31st, 2016): article

When I was in elementary school, I learned about dinosaurs and the asteroid that killed them and I was worried about that happening to us. But the more I read and the more I looked into the problem, I realized that it is so extremely unlikely that it isn't a threat. And in the case that there is a threat, NASA can now identify possible threats years out and move them out of the way years before they are aimed at Earth. Part of the reason we're so safe actually is Jupiter. It's so big that it sucks most of the stray asteroids into it and shelters Earth from being hit. I would say we are extremely safe right now.

There are comets or asteroids that we don’t know about. Programs such as LINEAR or PAN-STARRS use telescopes to detect them. Small comets or asteroids are not a problem for us, and fall apart when they hit the atmosphere Asteroids greater than 0.6 miles are a problem for us, because if they hit the Earth, they could stir up enough dust to block some of the light we receive from the sun. Our solution to this is to design large rockets that can collide with the asteroids, and divert the asteroid away from Earth. A good analogy is billiard balls. When you hit a moving billiard ball with another billiard ball, the first ball changes its course.