I got the part about increasing eventually, but I thought the third number also had to be the sum of the other two. I came up with the sum idea after trying (3 6 9), so only 2 tests. The idea that they had to be increasing came later. I don't have it open but I'm pretty sure one of my tests was (1 2 5) which should have tipped me off... in conclusion yes, I'm probably dumb.

I know you're joking, but I think this is important:

Failing the test does NOT mean a person is dumb. The point of the article is that confirmation bias seems to be a fundamental default in the way everyone thinks. Certain people with specialized training in inductive problem solving (scientists etc.) have learned to compensate.

If folks think it's an issue of intelligence, then they might be willing to think "but not me, because I'm smart." (After all, many programmers believe that they are smarter than the average bear). But while programmers are well-trained to think carefully about sequences of numbers, they might be as susceptible as anyone else to confirmation bias in other areas.

I wonder how much games like Twenty Questions plays into conditioning towards this kind of approach, since the implication there tends to be that the fewer questions you ask, then the better you've done.

Like the other comment said, this is not about being dumb. It's about human psychology: you'd rather not be wrong, so you jump at the possibility of being right, and "forget" to take the logical step of actively trying to disprove your rule (1 2 5 can be disregarded because you weren't trying to disprove the rule).