A pumps, the Colgan Dash 8 had two qualified crew who failed to note a speed decay and once it was pointed out to them by the stall warning system, failed to do anything constructive to recover. There have also been FOs who have silently sta there watching their captain fly them into the ground. So yeah, I think sometimes you can have a situation where 4 qualified crew could either not notice a speed decay or if they do, not speak up about it. People are unreliable machines.