On several occasions Liam Neeson pronounces words in a non-American fashion. Examples include anyone and everybody, where he pronounces the vowel in the second syllable "ih" rather than "ee"; and the converse for been, where he pronounces the vowel "ee" rather than the American "ih".
Two days after the break-in when agents in the DC field office are reading the front of the Washington Post, the date on the newspaper is wrong. It should be 19 June, not 20.
There is a reference to Richard Nixon being named TIME Magazine's "Person of the Year" for 1972. TIME Magazine did not use the title "Person of the Year" until 1999. Nixon would have been named "Man of the Year" in 1972.
During one of the drive-around scenes in the DC area, the car passes by the World War II Veterans Memorial, which did not exist in the time frame of the movie.
The 1-yr old National Museum of African American History and Culture is visible in scenes of Washington DC at the beginning of the movie, which starts in 1972.
At one point in the movie a character is seen wearing a pair of Ray-ban sunglasses with the logo printed on the yellowish tinted lens. Ray-ban didn't print the logo on their lenses until some time around the eighties.
Back in the early 70s, it was still quite common for people to wish one another a Merry Christmas or Happy Hanukkah and not the generic Happy Holidays that was said in the movie. People were not yet offended by the use of the actual holidays being mentioned as greetings.