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: He was written off the series, never to be mentioned again as if he never existed. As a matter of fact, according to the final episode, he never did exist.
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