Y2K. I thought I had a solid lower bound for the size of that one: Small businesses basically did nothing in preparation, and they still had a fair amount of dependence on date-dependent programs, so I was expecting that the impact on them would set a sizable lower bound on the the size of the overall impact. I've never been so glad to be wrong. I would still like to see a good retrospective explaining how that sector of the economy wound up unaffected...

First, the "economic collapse" I referred to in the original post were actually at least 6 different predictions at different times.

As another example, but not quite a "collapse" scenario, consider the predictions of the likelihood of nuclear war; there were three distinct periods where it was considered more or less likely by different groups. The late 1940s some intelligent and informed, but peripheral, observers like Robert Heinlein considered it a significant risk. Next was the late 1950s through the Cuban Missile Crisis in the early 1960s, when nearly everybody considered it a major risk. Then there was another scare in the late 1970s to early 1980s, primarily leftists (including the media) favoring disarmament promulgating the fear to try to get the US to reduce their stockpiles and conservatives (derided by the media as "survivalists" and nuts) who were afraid they would succeed.