Health warning: Healthy salad ahead. What surprising fact has recently contradicted one of your old beliefs?

Although I did study physics for a couple of years at university, my major was in philosophy. Nonetheless, I love science. If you want to know how the world works, what is in it and how the bits work together the good answers come from science, not from tradition, not from religion and not from philosophy. Of course, there are whole areas where science is not the useful tool for understanding: it cannot tell us why 1+1=2, or even what a number is, nor can it tell us what makes an act or attitude morally right or wrong, nor can it give meaning to human life.

 But even in the realm of human behaviour and relationships, science is a powerful tool to help us understand how we work: it is science that tells us that most of us are dishonest, but unhappy to admit that about ourselves, and that can help us understand why this dishonesty is so prevalent a part of human behaviour, even suggesting some solutions that just might work because they are solidly founded on verifiable evidence (Mazar, Amir & Ariely, 2008).

I have to admit that when I saw the New York Times headline "How Salad Can Make Us Fat," I suspected the sort of explanation for it, but that the mere presence of healthy salad on a menu can cause unhealthy eating was still surprising when backed up with solid evidence (Hutchinson, 2008).

Even though I'm fairly skeptical these days of all amazing claims, not believing something until there is solid evidence or sound reasons, I still regularly find that I have ideas accepted on little more than faith that is often proved undeserving of my trust. I'm reminded also of my surprise when I learned recently that the evidence suggests that is little health benefit in green tea, and a lot of health benefits in coffee addiction. I guess I'd just picked up that notion from friends, from social attitudes that are everywhere, and had not insisted on the evidence. The result was I was sincerely believing something wrong for years! Having the wrong idea about green tea probably wasn't very serious, but not understanding the way salads can work to undermine my health can be dangerous - happily, the article also suggests some healthy measures to counter our natural human tendencies (nature is so nasty!).

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My question is:
What surprising think have learned recently that forced you to reconsider some old opinions, that showed your old beliefs to be either seriously incomplete or totally false?

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Reference

Hutchinson, A. (2015, October 23). How Salad Can Make Us Fat. The New York Times. Retrieved from http://www.nytimes.com/2015/10/25/opinion/sunday/how-salad-can-make-us-fat.html

Mazar, N., Amir, O., & Ariely, D. (2008). The Dishonesty of Honest People: A Theory of Self-Concept Maintenance. Journal of Marketing Research, 45, 633 - 644. Retrieved from http://people.duke.edu/~dandan/Papers/PI/Dishonest_JMR.pdf