Corrected entry: In the scene where the comet hits the ocean, there is a massive traffic jam just 5 miles from the beach. Since everyone has known about the comet for over a year, wouldn't they have headed away from the ocean a few days/weeks/months earlier?
Corrected entry: Sara's dad put the key for the motorcycle in the jar 3rd to the right. When Leo "steals" the motorcycle he finds the key in the jar 4th to the right.
Corrected entry: At the end of the film, when Tea Leoni's character and someone are standing on the beach, waiting for the tidal wave to come, the tidal wave seems to make a miraculous stop, right before the beach, where as in reality, tidal waves go so fast, that once you've seen it, it has already hit.
Corrected entry: When the Messiah mission withdraws from the Comet to remote-detonate its nukes, the implication is that the nukes must be let off NOW to have any chance of success, and the Messiah is thus caught in the blast and damaged, conveniently putting it out of communication with Earth for the duration of its journey home. They are months away from impact and there would be time to fire up the main drive and make the return burn before detonating the nukes, thus putting a safe distance between themselves and the blast. The only argument against this is that if the attempt failed they might go back to the comet with the remaining nukes for a second try - this is never suggested, so why not get out of there?
Corrected entry: Leo reports his discovery to the local professional astronomer, who programs his software to plot a course for The Comet. It displays a three-dimensional graphic which shows the comet headed right for Earth. In his panic to report the discovery, he wrecks his car. First off, Leo should have reported the discovery to the International Astronomical Union's Minor Planet Center (http://cfa-www.harvard.edu/cfa/ps/mpc.html). We can forgive him this breach of professional etiquette since Leo wanted to report to his club's sponsor first. The real problem is that it takes several observations over many nights to get a good orbital calculation for comets, especially one that's a year away from impact. It's like looking at a snapshot of a baseball and being asked when it will hit the ground. What direction is it heading? How fast is it moving? One observation doesn't really tell you anything; you need to see it move. The 3D graphic was a bit over the top too. While not really impossible, it's more of a Hollywood silliness. It's there simply to drive the point home that we're in for a bit of trouble.
Corrected entry: In the opening scene, we see a group of young amateur astronomers stargazing. As we pan across the group, we see them studying maps with flashlights. The problem here is small: using a flashlight outside at night ruins your dark vision. The eye takes quite a bit of time to get adapted to the darkness; usually twenty minutes to get fully adjusted (less if you are in a light polluted area). Using a white flashlight destroys the very reason you're outside! However, the eye's dark adaption is not ruined by red light, so astronomers use red flashlights (usually modified by the high tech method of taping red cellophane over the business end of the flashlight). This may have been plot-driven; we'd want to be able to see the actor's faces.
Corrected entry: When the President is making his speech at the press conference he says that the as comet had been seen by a Young Explorer's Astronomy Club on a mountaintop in Arizona. Then when everybody's escaping from the coastal areas and it shows Leo driving down the highway looking for his girlfriend there's a sign for Virginia Beaches. Arizona is a pretty long way to go on a field trip, don't you think?