I used to be very often surprised by the English language (constantly running into my own false beliefs) and it was frustrating, until I started to read fan fiction. Since then, my attitude changed from 'wow, it's a word' to 'ah well, if t ain't broken, don't fix it'. Not sure if it is a good thing, but it seems to help with communication.
ETA: having just read the article 'Why don't people help others more?', I begin to suspect that donating to charities is actually one of the things that I don't want to do at all, even though I allocate money for it in my budget. I always thought of myself as not stingy, and it turned out I am. Feels... odd.
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.