It's probably every pilots greatest fear - catastrophic structural failure. And I sometimes think to myself flying a 57 year old aircraft "hang on a minute, how do I know my main spar doesn't have a crack in it?". I don't, of course.
But I do know that aircraft get inspected every year very thoroughly. I also know that long before metal cracks, it has to reach its so called plastic state, which means deformation, which is often detectable to the eye. This doesn't mean there can't be cracks that are not detectable, of course. But a crack doesn't always mean a structural failure is imminent either. I also take courage in knowing that a member of our owner's club, who operates 30 of these in his night freight operation, regularly puts 15.000hrs on them or more. So I have some 10K hours still to go, I suppose.
We've all seen the horrific Hercules crash and the failure of the spar box on that:
In the Hercules case the plane was also at max gross and the pull-up after the water release probably took it over Va. But nevertheless, an undetected crack hidden behind a doubler was the main culprit.
My aircraft has a cold formed one piece main spar that runs the length of the wings. It also has a smaller spar behind it, but I doubt it could take the entire load should the main one fail. Still, it adds some extra security. Many designs use more than one spar and if you're extra nervous, seek out aircraft that employs this design.
As a complete side, the new composite spar structures are in a way much more insidious as they're virtually impossible to detect cracks or internal debonding in. At least with metal you can spot trouble as long as you inspect properly. So in a way, I feel a lot safer in a 57 year old metal plane than I ever would do in a 57 year old glass fibre one.