Character mistake: Both in the title and throughout the episode, people refer to 'Lord John Marbury'. However, the first names of peers are not used when also using their titles; so Marbury should be called either Lord Marbury (more accurately Earl Marbury), or just John Marbury. A minor error in the real world, but the White House is supposed to be aware of diplomatic eccentricities like this.
Character mistake: CJ says that Marbury was ambassador to either India or Pakistan while briefing someone on the phone. Britain doesn't have ambassadors to those countries - being part of the Commonwealth they have High Commissioners instead. A minor error, but one the White House - versed in diplomatic protocol - would be aware of.