Character mistake: Previous episodes have established this story arc to be occurring in the year 2154. Yet here, when T'Pol records her log, she cites the date as 2152. (00:32:00)
Day 4: 10:00 P.M.-11:00 P.M. - S4-E16
Character mistake: 23rd minute. Jack meets the female assassin in Anderson's apartment. She demands that he show his credentials. Jack says he's with CTU on a provisional basis and therefore he has no badge. The agent that came with Jack confirms that. Yet, in the 13th episode (minute 14) when he enters the sports shop with Audrey's husband, he shows his CTU badge to the owners in order to prove that he in fact is a federal agent.
Character mistake: In season 4 episode 13, "Tuesday," when Marshall is meeting with Ulrich, he is both able to understand and reply in German. In season 5 episode 15, "No Hard Feelings," Marshall almost misses the opportunity to change the street light from red to green, because the text is all in German. Jack has to tell Marshall which word is green, indicating that Marshall does not understand the German language.