From time to time, a producer will insert footage from another movie or TV series into the show he/she is producing, usually as a cost-saving maneuver. (Irwin Allen did this EXTENSIVELY in his Sci-Fi Series). Sometimes you can identify what scene was "Borrowed" from the other series. Yesterday I was watching an episode of Lassie from the tenth season called "The Disappearance" where Timmy is separated from the Dawg when a storm capsizes the Boat he and his Father are on. There is a scene where Timmy and his Father hire a Pilot to fly them over the lake in search of Lassie. Look closely at the plane, does it look familiar? It's the same plane the skydivers use in Ripcord and it has the parachutist logo on it!

Anyhow what other instances of Stock Footage have you recognised when watching a show?

The opening sequence of Married... With Children uses exterior shots of Chicago from National Lampoon's Vacation.

an episode of the short lived 1960s series "THE TIME TUNNEL" (which was about 2 guys trapped in time-it was similar to "QUANTUM LEAP") where they were on the Titanic used footage from the movie "A night to remember" which was a 1950s movie about the doomed voyage of the ship.