As Napoleon reportedly said, P(incompetence was the cause | bad things happen) > P(conspiracy was the cause | bad things happen).

There is no non-conspiracy hypothesis regarding 9/11. No one thinks it was an accident. :)

Actually a lot of people think that the non-prevention of 9/11 was an accident due to incompetence and specifically a lack of data-sharing among intelligence agencies. Of curse, there was a lot of guilt-induced hindsight bias at the time. Meanwhile, some of the conspiracy theorists were claiming that the government could have stopped the attack but decided not to. In this case, incompetence seems the likelier cause, like Napoleon said.