(28:28) The car shown is the newer model of the one that Gus actually owns. However, in the next scene the correct car is shown.
The British ambassador gives Shawn diplomatic immunity. This is wrong on three counts: No citizen can ever have diplomatic immunity within their own country; not every embassy employee gets immunity; and ambassadors cannot confer it, only the host government.
When administering the polygraph Lassiter asks Shawn "What is your name?" and "where were you the night of the party?". On a polygraph, all questions must be asked in such a way that they have simple Yes/No answers. In fact, only the last question is a validly formed polygraph question.
The son of the British ambassador states that the British embassy is in New York. The British embassy is in Washington DC. There is only a consulate in New York.
In the picture of the man ascending the staircase at the ambassador's house the man has his hand on the handrail and it's presumably Shawn. However, when Shawn actually ascends that staircase in the beginning of the episode he doesn't use the handrail.