ISTR the top spar of the horizontal stab had a known problem with cracking around one fastener. The NDT procedures used at the time to monitor the crack and inspect the centre and lower spars was not able to detect a crack in the centre spar. These cracks caused the stab to fail and fold up against the vertical stab.
Inspection of the worldwide fleet of high time Boeing 707s found several more about to fail. There was a mod. program that involved strengthening the centre spar, stress relief shot peening the upper and lower spars and replacing all the fasteners with Hi-Locks.
I worked on one of these mods at BCAL around 1981. Several weeks of long nights I think.