We humans are absolutely *terrible* at generating randomness. Case in point. You're asked to press the "f" & "d" keys randomly. After a few presses, a simple probabilistic model is likely to be able to predict your next key with a 60+% accuracy!
Try it here: people.ischool.berkeley.edu/~nick/aaronson-oracle/
Code: github.com/elsehow/aaronson-oracle
I thought I was better, I'm not.
(This BTW is known as Aaronson Oracle).