Visible crew/equipment: When Don comes into the garage to apply for the job, the T-mark can be seen on the floor by Fonzie's feet.
Revealing mistake: When Arnold's burns down you can see the set walls wobble a lot.
Ralph's Family Problem - S7-E25
Factual error: Fonzie takes Ralph on a motorcycle ride to make Ralph feel better, but they are roaming through hilly areas. There are no hills in or near Milwaukee.
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.