I came across this documentary, where two ex-boeing employees say that 737NG's are build with ill-fitting parts. they talk about the cords that constitute the airframe skeleton, which are produced by a third company sub-contracted by boeing. these parts are supposed to be produced by computer controlled machines, instead they are made by hand. the problem with this is that they are not as accurate as they should be, which makes the whole airframe not being strong enough and not as safe. previous generations of 737's were build by hand made parts but the NG series were certified to operate at higher weights, speeds and altitudes because these accurate parts can fit tightly to make a stronger aiframe.
i just hope this is not true at all.