People are very bad at being intentionally random.
I remember hearing some anecdote about some professor doing a "test" to see if his class was psychic. He told then he'd picked out a sequence of something like 8 random 0's and 1's, and had the class try to guess what he had written down. He had them all raise their hands, he'd start calling out the numbers, and they'd lower their hands once they had gotten one wrong. By statistics, you'd expect half the hands to go down each time, but it didn't. After half the numbers had been read out, which was 1011, a much larger number than you might expect still had their hands up. Aha! the class was psychic.
Then the professor read the rest of the numbers, 0000, and the remaining hands went down quickly. Their psychic abilities failed.
What happened? Well, if you ask people to pick a random series of 4 1's and 0's, they don't produce a random list. Almost nobody picks 0000 or 1111. 1011 and 0100 are very common, since people start out alternating, but then decide they need to do some repeats because repeats sometimes happen. The net results of people's "random" picks are statistically very skewed, and all the professor did was take advantage of that fact.
As a side note, it is possible to train people and even animals to act more randomly.
