For the famous "mirror reflections" in which Sam sees who he leaps into, the series used the old trick of a dual set with a clear glass in the "mirror". Scott Bakula would stand on one side and the actor playing the person he leaped into on the other. If you look really close at the glass, you can see sometimes Scott Bakula's reflection. (Especially if the mirror is near a source of light like sunshine).
In the Season 5 opening, "Theorizing that one could time travel within his own lifetime...", the visuals reflect the narration. The narrator says, "Dr. Sam Beckett stepped into the Quantum Leap accelerator and vanished...", what is seen is him leaving the imaging chamber after leaping back to New Mexico.
In the first season, Sam mentions having a sister named Katie. In the 2-part pilot "Genesis", Sam mentions that she married a Naval Officer named Jim Bonney. However, in the first season episode "The Kamikaze Kid", which hinges on Sam stopping a sister from marrying an abusive boyfriend, Sam mentions that Katie married and abusive alcoholic, later named "Chuck" in the "Leap Home" episode. It is very likely that Katie married Jim after leaving Chuck. Also, in "Disco Inferno," Sam mentions that he has a brother named Tom and that he died in Vietnam. He mentions this again in the episode "Animal Frat" before finally seeing Tom again in "A Leap Home."
Ziggy's opening narration says that "only Sam can see and hear" Al which was thought to be true for the first season and for the first twelve seasons of season 2, but Sam and Al discover in Quantum Leap: Another Mother - September 30, 1981 that animals, very young children, the mentally disabled, and people near death can also see and hear him (as well as seeing Sam's true form).