Rule 10 is referred to by Gibbs in Ziva's office. Rule 10 is never get personally involved.
Bishop mentions that in one of her journals, Ziva wrote "happy as a oyster" instead of "happy as an clam". During her run on the show, a running gag was Ziva's difficulty with American idioms, resulting in mix-ups like "donkey's butt" instead of "horse's ass" and "eager platypus" instead of "eager beaver".