I went on the Boeing visitor tour in 1983 IIRC, rode the elevator up to the 747 assy observation platform, etc. - Still an amazing sight!
But outside the building was the 767 test article in the fatigue test rig. Over and over, the wing was flexed to the cert limit, then relaxed back to unload it. It didn't break while we were there, and I'm sure they interrupted the test every couple hundred cycles for routine inspection.