It may well have been the actions of an Orlando border agent Jose Melendez-Perez, an Army veteran who took protection of the United States seriously. The individual in question had incomplete documents, and so was interviewed by the agent. His story did not add up. He wouldn't say who he was to meet, and how he was getting around with his limited English skills. He did take the finger prints of Mohammed al Qahtani before denying him entry. He was given they opportunity to return to Dubai, paying for his ticket back. Facing this or imprisonment he took the opportunity.
The day of the 9/11 terrorist attacks, Flight 93 only had four terrorists on board, not five like the other planes. Perhaps one less man defending the cock pit after taking over the flight is what allowed the heroes aboard Flight 93 to drive it into the ground instead of the intended target in Washington, D.C.
After the attack on Afghanistan, Qatani was captured in the Tora Bora area. He was identified from the finger prints. He had not given his name when captured, but the finger prints gave him away. Qatani ended up at Guantanamo Bay.
There he was subjected to accepted interrogation procedures, restraint on a swivel chair, deprivation of sleep, loud music, prohibition of praying, threats of rendition to countries who use torture. After a week the interview began. He was accused of being the twelfth hijacker. He was asked the location of Bin Laden. He was accused of wasted the interrogators time, and asked to give one name. He gave Abu Ahmed al Kuwaite, who taught him internet.
Some months later, that name came up, Sheik Al Kuwaite. Invasion of the compound where he was located lead to the killing of Bin Laden.
Small things can bring important results.