Roughly 30 years ago, while flight testing an E6 (a military version of the 707 with CFM engines), they suffered serious vertical tail damage. I don't recall what they were doing at the time (it may have been flutter testing but it's been a
long time so the memory is fuzzy).
What's not so fuzzy is that I happened to see that airplane parked in a hangar at Boeing field shortly after the incident. Roughly half the vertical tail and nearly all the rudder were missing
I don't seem to recall anyone ever claiming that the 707 airframe wasn't robust.
