In engineering, that sharp corner (or groove in the Kit Kat) is called a "stress concentration," a spot where the shape of the object makes it more likely to break under stress.
You don't want the red bit.
So if you're an airplane maker, how the hell do you fix that?
Well, have you ever noticed how on every plane you've ever been on, the windows you look out of have rounded corners? Those curves are pretty much the only thing keeping the plane from tearing itself apart in midair like in that scene from Fight Club. It distributes the stress to all of the various points along the rounded curve, rather than on that one sharp corner, which otherwise would (as they found out) tend to pull apart and form a crack over time.
Trust us, this was not easy to figure out. Experts had no idea why the planes weren't holding together until they tested the structure by simulating the repeated pressurization of the cabin. Sure enough, the fuselage eventually burst like a bootleg condom, and the break started with cracks right at those window corners.
Getty / plane-crazy
Top: Safe and enjoyable ride. Bottom: Explosive Caesarean section.