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.
Obamacare, an excellent idea and long overdue, but implemented and deployed in the worst way possible, is a typical example. The Ebola crisis response is another. Handling of the Snowden affair... Take almost any issue, political or economic, international or domestic, and it has been botched pretty bad, not out of malice, but out of incompetence. Well, maybe the Quantitative Easing is an exception, I am not qualified to judge.
http://www.vox.com/2014/5/20/5732208/the-green-lantern-theory-of-the-presidency-explained
Summary: The President doesn't have all that much freedom to control the government. The office was designed that way.