It is rumored that Burt Lancaster had in his contract that his gymnastic equipment be shipped, set up and struck at each location so he could work out to keep in shape.
The film crew had to obtain a special authorization before shooting in the CIA building. Burt Lancaster asked Senator John V. Tunney, who then asked a CIA head director to authorize the shooting. Although filming and recording was not allowed in the building, Michael Winner did it anyway. Plus, no identification of any kind was asked for the crew members, but they had to wear badges made by the CIA especially for them, and with a scorpion on them. Those badges had to be destroyed after the shooting.
The production company was filming in Washington, D.C. and was staying at the Watergate Hotel during the time of the infamous break-in at the Democratic National Committee offices at the Watergate complex on June 17, 1972.
When Cross first gets into a car in Vienna, they are in front of the same doorway where Harry Lime makes his famous appearance in The Third Man (1949).