Those very un-aerodynamic rows of bolts at the wing butt joints used to be covered with a neat cover strip in times of olds.
Problem was, when they finally corroded and failed, they would quietly slip unseen overboard beneath the cover strip. This would not be visible on a regular visual inspection.
The cover strips were finally removed after a fatal wing separation after too many bolts had disappeared, leaving too few to carry the loads.
Regards,
Old Smokey