For example I wasn't really surprised while reading HN featured article "The healthiest people in the world don't go to the gym" [4], because the results of research presented there were close to my expectations (due to the model that emerges from all my previous knowledge).
On the other hand Libet's experiment [3] regarding consciousness was very surprising for me - because it's results are counterintuitive and if I was asked to guess them I probably would't be right.
Which facts are/were most surprising for you?
[1] https://en.wikipedia.org/wiki/Information_content [2] https://plus.maths.org/content/information-surprise [3] https://qz.com/quartzy/1452630/the-healthiest-people-in-the-world-dont-go-to-the-gym/ [4] http://www.informationphilosopher.com/freedom/libet_experiments.html