Continuity mistake: When they are on the motorcycle near the end, he is first traveling with the flow of traffic that is stopped (west). When she gets on the bike with him and the baby, they keep going with the flow of traffic. Than the smaller asteroid enters the atmosphere, it is traveling in the same direction they are driving on the motorcycle. But when it passes the girl's family in the van, now the asteroid is traveling against the flow of traffic (east).
Factual error: The shock wave that hit Messiah when it detonated the nuclear bomb on the comet would not have existed in space. It may have been hit by debris, but that would have destroyed the craft.
Factual error: A wave 1000' high would have obliterated the containments and wiped out the spent fuel pools of every nuclear plant on the east coast. Even if they were all shut down, there would be no time or way to remove all the fuel and no place to safely store it all if they did. There are thousands of metric tons of fuel that have to be actively cooled for years, otherwise it would all catch fire and burn, or at the very least boiled all its cooling water away and melted/uncovered it to allow gamma shine. This would have, in and of itself, made the northern hemisphere uninhabitable for several hundred human lifetimes.
Continuity mistake: When the boy takes the motorbike from the garage the mirrors are out of line. In the next shot before he rides out, the mirrors are all in line.