So we have a perfectly servicable aircraft airborne with the outflow valve open. Happens all the time. Warning horn sounds to alert the crew. Two options for this horn, 1 take off config, well they were airborne, 2 pressurisation! You might think that this would be worth checking, but no 'they were confused'! Sounds like total incompetence to me. Of course Boeing and the airline will have to face months of litigation. Another crew c..k up.