When Beckett and Castle go to the library and finds Tolstoy's War and Peace, it's not in the Russian Literature section but in the Danish section, as the books around it are Salomonsen's Konversations Leksikon and Københavnerliv 1883-1912.
When Castle and Beckett get to the Russian literature section of the library there is a book on Copenhagen's history from 1883-1912. Copenhagen is the capital of Denmark.
In Principal Duncan's office, when Castle and Beckett are accusing the killer, the student's princess sash is over the wrong shoulder so that the writing is upside down.
Both Castle and Beckett refer to the Principal as 'Dunnan' when his name is Duncan.