Three Thalmor at the Embassy can respawn when killed during Diplomatic Immunity. It looks like they have an imbedded respawn flag deep in the various sets of actor base templates. Although you shouldn't be able to re-enter the Embassy, they either attack you through openings in the walls or somehow manage to open the main gate. The Thalmor Embassy is supposed to be a no-reset zone, but since these three are placed in one or more exterior cells they respawn due to an engine bug.
FormIDs: 0006B3C2, 0006C0B5, 000BCD05