After the Hanssons take the O'Neills to Mass, Bonnie Hansson asks Juliana if "...she (Juliana) enjoyed the service?" As a strident Catholic, Bonnie would have said, "Did you enjoy the Mass?" Protestants have a "service"; Catholics have a "Mass".
Signs on one of the Washington, D.C., metro stations used in the movie read "Archives-Navy Memorial-Penn Quarter," but the words "Penn Quarter" were not added to the station name until January 2004, three years after the movie takes place.
When Hanssen's car is thoroughly inspected his trunk contains several assault rifles including German Heckler & Koch G36s. The film takes place early in 2001. HK did not announce the release of the G36 until Spring 2001 and did not begin releasing free LEO (Law Enforcement Only) samples until 2002. It is also unlikely that Hanssen would even be issued such as gun as he did not test new firearms for the FBI.
Eric and Robert enter a church and Eric makes the sign of the cross incorrectly. Since much of the plot involves Eric's and Robert's Catholicism, it would have been an error that would have made Robert suspicious.
When Hanssen and O'Neill first enter the chapel together, Jesuit-trained O'Neill performs the sign of the cross incorrectly, touching forehead-shoulder-shoulder-sternum, rather than forehead-sternum-shoulder-shoulder.
The Dell laptop and LCD monitor that Robert has in his office, when he shows Eric a movie clip with Catherine Zeta-Jones, was manufactured after 2001. While driving on Wilson Boulevard, the license plate on the back of the black SUV Eric is driving is a Virginia plate. In the traffic jam in the next scene, the license plate on the front of the SUV is clearly a Washington DC plate.
When Kate is explaining the details of his assignment, she has the pager sitting on the table. When she tells him about it, she is holding it. The camera switches to Eric's perspective and she brings her hands up from under the table to slide the pager over to him.
When Hanson and O'Neill are driving on the highway, the Kennedy Center For The Performing Arts appears almost directly to the left of O'Neill. The screen flashes to Hanson, then when it goes back to O'Neill, the Kennedy Center is farther away, as if they are just now driving toward it.
When the agents are disassembling Hanssen's Taurus, they are seen removing an alloy wheel from the driver's side front. However, the vehicle is a Taurus GL which had steel wheels with plastic covers seen in the rest of the film.
When Eric O'Neill removes Robert Hanssen's Palm Pilot from his briefcase you can clearly see the Palm Pilot Model is a Palm III, which doesn't use an SD memory chip, which is lying next to it. This Palm has a 8 meg of internal memory only - no External memory card.
In the long shot of the scene of a traffic jam on Memorial Bridge, no-one is in the driver's seat of any of the cars. This is probably because those cars are parked, not stuck in traffic. On that part of Rock Creek Pkwy in DC, there is parking on both sides of the street. There is only one lane of traffic in each direction, with room for cars on both curbs for parallel parking.
When Eric looks at the Internet history on Hanssen's computer, one of the URLs displayed uses incorrect syntax: "www.church/democrats/clinton.com". The slash character is invalid in a top level domain name (www.imdb.com for example). However, this simply means the URL was incorrectly entered, which is also maintained in Internet Explorer history.
The metro station labeled as "Archives-Navy Memorial-Penn Quarter" that Kate exits when meeting with Eric is actually the Federal Triangle metro stop. The adjacent Post Office Pavilion is visible in the background.
When Robert Hanssen is making his final "dead drop" of government secrets in a metro DC park in February, the sound track includes the distinct sound of robins singing. Robins do not migrate into the DC area until many weeks later.