The Ballad of Joanie and Chachi - S11-E2
Visible crew/equipment: After Chachi walks out of the college classroom, when Joanie takes a seat and starts crying the boom mic dips into view at the top of the screen, in the wideshot.
The People vs. the Fonz - S11-E11
Factual error: When the Cunninghams are talking about Fonz's trial, Mr. Cunningham can be seen wearing a digital watch. Digital watches didn't exist in the 50's.
Visible crew/equipment: When Fonzie and Chachi are wearing their tuxes, Chachi nervously picks up the telephone for no reason, and just as he walks around the table we can see set lighting at the top of the screen, over the window.
Factual error: Joanie, looking at an issue of Billboard magazine, says, "Chachi's record has only been out two weeks and it's already #178 on the chart." Since 1959, Billboard has had a weekly "Bubbling Under" chart in addition to the Hot 100. In the 1960s, when this episode takes place, the Bubbling Under chart listed no more than 35 songs (#101 to #135). There is no way Chachi's record could be #178.
Continuity mistake: A pilot takes Chachi up in a small plane so Chachi can go skydiving. In the long shots showing a real airplane in flight, the sky is deep blue and cloudless. In the close-ups, filmed in a studio, the plane flies through several wispy clouds, likely formed by a wind machine blowing the mist rising from dry ice.
Answer: When it was originally broadcast there was a different ending. A new ending was added when the show was rerun to set up the Mork and Mindy series.