A day after the terrorist attacks of September 11, 2001, director Roger Michell had the World Trade Center towers digitally removed from the opening main title sequence in the film. In the DVD commentary, he admitted that it was a mistake to erase them, and make it appear as if they did not exist. During the re-editing of the film, Michell reinserted them as a tribute.
A young Bradley Cooper was in a scene that was supposed to take place early in the movie, just after the first scene of Ben Affleck's character in the Metropolitan Museum. He played a young lawyer getting a job interview. This scene was deleted from the movie, but can be seen on the blu-ray, in the Deleted Scenes section.
In order to attract a wider audience, the theatrical trailer was arranged in such a way as to make the film look like a high octane thriller, as opposed to the rather slow moving drama it actually was. This move was heavily criticized for misleading moviegoers.
The accident scenes were shot in two days, with the F.D.R. highway shut down to regular New York City traffic.
It was Samuel L. Jackson's choice for his character to wear a purple hat. Jackson's characters often feature the color purple: Mr. Glass wears purple clothing in El protegido (2000) and Glass (2019); and Mace Windu has a purple light-saber in Star Wars. Episodio II: El ataque de los clones (2002) and Star Wars. Episodio III: La venganza de los sith (2005).