During the standoff at the police station, Ziva's revolver doesn't have a hammer.
When Ziva sits at her old desk, she looks across the way and envisions Tony sitting there. However, the clip shown of Tony sitting at his desk is from the Season 1 episode "Dead Man Talking," which took place prior to Ziva's arrival to NCIS.
Vance says all 538 members of Congress have been placed into protective custody. Congress is comprised of 441 Representatives and 100 Senators for a total of 541, not 538. There are 435 voting members in the House, and 6 non-voting representatives, one each from D.C., U.S. Virgin Islands, Puerto Rico, American Samoa, Guam and the Northern Mariana Islands. The number 538 is the number of members of the Electoral College, one for each representative and senator plus three members for the District of Columbia.