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.
Let's say you see someone who gives 25% of their income to the charities GiveWell recommends and says they do this because they think it's the right thing to do. This is enough money that if they're optimizing for your own happiness, social status, long term welfare, or pretty much anything else about them there are almost certainly better ways they could spend it. I guess you could say "they're not being altruistic, they're doing a poor job of acting in their own self-interest" but that seems like a pretty big stretch.
Doesn't matter what "they say" in regard to what the right thing to do is, right? It only matters if the act is pure altruism—that is, the giver gets no benefit, or even sacrifices, for the recipient.
My understanding (a la The Selfish Gene) is that the replicating genes of humanity will benefit from even fully anonymous gifts to people we've never met. Altruism is actually self-interested at the gene (or "replicator") level, no matter how purely altruistic (or 'selfless' and 'moral') an action may seen at the level of the organism.