Keep Circulating the Tapes: Many fans didn't believe the show would ever be put out on DVD. And while the entire animated series was eventually released on DVD in the early 2000s, within the last few years, Bandai Entertainment decided not to retain the rights to the show.
Althought the pages are not in English, the manga can be found here with summaries in English.
The Other Darrin: A rather extreme, albeit brief example, occurs in the American dub. According to several sources, the entire primary cast had head colds when recording the first couple of episodes, so the studio had to use temporary replacements. As a result, fans heard a rather obvious Vocal Evolution between episodes 1 (primary cast), 2 (the replacements), and 3 (primary cast).
The OVA series also makes do with replacing a few of the characters' voices.