The fuselage of the McDonnell Douglas DC-9-80 (also known as the "MD-80") has a pronounced cusp at floor level, as can be seen in this diagram (courtesy of Boeing, eater of McDonnell Douglas - at least, according to @JWalters):
Although the DC-9-80/MD-80's fuselage cusp is nowhere near as extreme as the Boeing 377 Stratocruiser's, it still seems like an unnecessary stress concentrator with no apparent reason for being there; why did McDonnell Douglas put in the cusp, despite its potential for accelerating fatigue damage to the fuselage?