One of the smartest people in my high school spent a class arguing that a there were 4^20 possibilities for a sequence of 4 amino acids, when in fact it was 20^4. Not quite as elementary as yours, but our brains all play tricks on us.
Wikipedia says there are 500 known amino acids. I take it you were talking about a domain involving fewer potential amino acids?
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.