---I would think think the failure mode would be flutter or divergence, as the 747 exhibited a Mdf of M 0.998 [flown by the OEM] so it does have a controllability at high Mach numbers.
as mentioned above the DC-8 was the first civilian airliner to go sonic [which is actually extends a little above Mach 1.000]. this was done in a high speed dive from 50000 ft. in {I believe ] 1953
The Boeing 727 also had an interesting excursion to Mach 1.?? after someone tried to emulate Ol' Tex