My own example: I used to be a pretty strong believer in the strong crisis version of peak oil. This was mainly because I was around people who were; I went along with fairly naive trend extrapolation (exponential resource use increase, hard to develop new resources).
Eventually, around 2012 or so after oil prices had spiked then dropped then risen again and shale oil was a thing, I tempered my ideas, and took the much more rational view that oil wouldn't have a crisis-like peak, more a plateau. I believed that extremely high oil prices could drive massive adoption of much more advanced drilling methods. Therefore there would never be a crisis of extraordinary oil scarcity, but we could very well face $200/bbl oil (which we'd learn to live with like $140/bbl oil). But I didn't believe oil would really drop; I was bullish on almost all resource prices and on developing nation growth driving them.
While this was founded in a better understanding of economics and the petroleum industry, it was still grounded in false assumptions of continuing high-pace developing world growth and pessimism about alternative energy. Oil has now plummeted to $80/bbl, which I never expected to see again in my lifetime.
Same thing here from around 2003 to 2006. I did not see the oil shale boom coming. I found plausible all of the peak oil pundits who argued that oil shale would barely, if at all, have an energy return on energy invested (EROEI) greater than 1, and thus it wouldn't matter how high the price went - the costs would keep pace with the revenue, and it would not be economical to develop it. Of course, those pundits turned out to be wrong.
I remember the day when I really started to doubt peak oil. It was when I saw a TOD article on Toe-to-Heel-Air-Injectio...
What is something you used to believe, preferably something concrete with direct or implied predictions, that you now know was dead wrong. Was your belief rational given what you knew and could know back then, or was it irrational, and why?
Edit: I feel like some of these are getting a bit glib and political. Please try to explain what false assumptions or biases were underlying your beliefs - be introspective - this is LW after all.