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.
I think that when you start reasoning about quantum foundations it should be remembered that you're leaving the boundary of testable physics. This is to say that even if you've concluded that many-worlds is most likely to be correct with your current information, that there should remain a pretty high degree of uncertainty in your conclusion.
It has been shown experimentally long ago that MWI requires full Quantum Gravity, not just Quantum Mechanics (plus Newtonian gravity or General Relativity, or even semi-classical gravity).
EDIT: provided an alternate link (paywalled, sorry).