Jennifer Anniston is in one episode. She is the love interest when Sam plays a Vietnam vet who is paralyzed from waist down.
The hit TV show Quantum Leap (1989-1993), also produced by Donald P. Bellisario, had plans to have the main character Sam Beckett (Scott Bakula) leaping into the body of Thomas Magnum! Magnum P.I. cast members John Hillerman, Roger E. Mosley and Larry Manetti were also slated to appear. Plans for a Magnum P.I. movie in the early 90s (which never materialized) led to the early demise of this episode plot, but some footage was filmed, namely the end-of-episode "juMagnum P.I.ng in" scene in which Beckett jumps into Magnum's body, turns to the camera, and raises his eyebrows the same way Tom Selleck made famous. If the episode had aired it would have created a bizarre continuity error, because Magnum P.I. had already been established as a TV show in the Quantum Leap "universe"! To add to the confusion, in the Quantum Leap pilot, Sam Beckett mentions that his sister is married to a man named Jim Bonnick (never seen). Jim Bonnick is the "Mac" look-a-like character (Jeff MacKay) in Magnum P.I.. Quantum Leap never clarified if this was supposed to be the same person or not, however.