For some context, my family members often browse alternative media where different kind of specialists and intellectuals voice their opinions on current state of the world and it’s future, but I noticed it’s mostly speculation with little evidence and shouldn’t be considered a reality. That’s why I’m looking for popular or not-so-popular ideas and speculations from the past that turned out to be untrue but a lot of people actually believed it. I guess there are more examples in modern history because of television and the internet, but I’m sure that kind of thing also happened in medieval or ancient history.
While you wait, here is an answer from /u/wotan_weevil about the actual scope of the Y2K problem. Y2K was an actual problem that was largely avoided due to a huge amount of behind-the-scenes labor by computer programmers, although the actual extent of possible problems is less apocalyptic than media makes it out to be.