According to Robert H. Justman in his book "Star Trek: The Real Story", this show's famous theme was not the first one written. Lalo Schifrin had written a main theme, but creator and executive producer Bruce Geller decided that it was inappropriate. Instead, Geller used some chase music Schifrin had written for the end of the first episode. That throwaway musical cue became one of the most famous and recognizable television show themes in history.
Even though Rollin, Cinnamon, Paris and Willie were somewhat famous for their various professions outside of their work with the IMF, no one ever recognized them during any IMF missions, even the ones they undertook within the United States.
When the reel-to-reel tape recorder is playing the mission's instructions, it is actually in a "rewind" mode rather than a "play" mode. This was done because the tape moved too slowly to be believed when it was "playing".
In numerous episodes a very tiny black "tape player" is used. It is identifiable by the aluminum tape reels which each have three holes. This was a dummy prop and, in several shots, it is obvious that the tape is a continuous loop running around both reels since the shiny aluminum center of the "takeup" reel is visible rather than the brown tape color.
In almost every episode someone has to pick a lock. They insert the pick, wiggle it a few times and open the door. To actually open a lock you must also rotate the cylinder just as you do with a key. For this you would use a second "L" or "Z" shaped pick to exert pressure while you moved the pins with the first pick. This is never done.
Voice on Tape: As always, should you or any of your IM force be caught or killed, the Secretary will disavow any knowledge of your actions. Good luck, Jim. This tape will self-destruct in five seconds.