Probably not too interesting, but after studying physics at university I was pretty sure that the Many-Worlds interpretation of QM was crazy-talk (nobody even really mentioned it at uni). Of course I didn't read Eliezer's sequence on QM (although I read the others). I mean I had a degree in physics and Eliezer didn't.
Then after seeing it over and over again on LW, I actually read this paper to see what it was all about. And I was enlightened. Well, I had a short crisis of faith first, then I was enlightened.
This all could have been avoided if I had read that paper earlier. The lesson is that I can't even trust my fellow physicists :(
I find Eliezer's insistence about Many-Worlds a bit odd, given how much he hammers on "What do you expect differently?". Your expectations from many-worlds are be identical to those from pilot-wave, so....
I'm probably misunderstanding or simplifying his position, e.g. there are definitely calculational and intuition advantages to using one vs the other, but that seems a bit inconsistent to me.
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.