The little girl Sadie rattles off a long fact-filled dissertation on "Canadian" geese. Anyone with her level of interest would know there is no such bird. They are Canada geese.
The experts in the film continually refer to the asteroid as a brown dwarf, a remnant of a dead star. In fact, a brown dwarf is a body which, although very large, is still too small to ever actually ignite and become a star in the first place.
The plot hinges on the relatively small piece of the asteroid which remains embedded in the moon being of very high mass, due to its high density as a piece of a brown dwarf. In fact, the only reason the density of a brown dwarf is so high is because of the dwarf's large mass, which generates such a high gravitational field it compresses the mass. If a piece of it is removed from the gravitational field, it will expand until its density is that of normal matter, much as compressed air taken out of the tank no longer remains compressed.
When the piece of magnetized brown dwarf came close to the meteor field, meteors which are composed mostly of iron, would have been drawn to it and thus increased its total mass.
When the orbit of the moon changed, it's rotation should have changed too. At some point the other side of the moon should have been visible to Earth.
The asteroid crashes into the moon in a fiery blaze. Since there is no atmosphere there, fire is impossible on the moon.
The Kinter family is from the US, but their cars have Ontario license plates.
After the impact on the Moon, a planet killer sized chunk is headed to Earth and due to hit in 14 hours. When then next problem arises, nothing more is heard of the planet killer.