What false beliefs have you held and why were you wrong?

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.