A line was edited in the President's press conference scene. President Beck stated "Life will go on, we will prevail." Originally, President Beck said "Life will go on, we will prevail. This is not Armageddon." The producers later realized that the movie was going to be in box-office competition with the movie Armageddon (1998).
After discovering the comet, one of the astronomers is killed in an automobile accident. This mirrors the real-life automobile accident death (July 18, 1997, in the Australian outback) of astronomer Eugene Shoemaker, who helped discover the Shoemaker-Levy 9 comet that collided with Jupiter in 1994 and was a source of inspiration for this film.
Morgan Freeman wanted his character (President Beck) to be wearing an earring. Director Mimi Leder turned him down. Later, we see the President addressing the nation from the oval office. His sleeves are rolled up, and one of Freeman's tattoos is showing. The director liked this. She felt it gave the President an everyman look.
At the beginning of the movie, when Dr. Wolf is using his computer, the screen reads May 10, 1998, the date of the film's release.
During the school assembly, one student makes the observation to Leo Biederman, "You're going to have more sex than anyone else in our class!" This line was improvised by Jason Dohring, and the reactions from the other students are genuine.