See those little teeth in the middle? Those used to be full splines.
I guess I was lucky because the last of the splines gave way in my garage and not 100 miles away from home. The nut got loose or when I switched to acorn nut to use the road side tool that acorn bottomed out. (I thought it was deep enough.) The other somewhat frustrating thing about this nut is the conflicting advice I have gotten on how to tighten it. Lummie says a little blue loctite and torque it. Ed says don't loctite it or it will get stuck, so just hit it with an air impact going on. I honestly hate the idea of using an impact gun to install anything. I am pretty sure I need to shim it some and this loose/non-shimmed nut could have been contributing to the engine's crank seal leaking issues as well. I ate up most of my rotor teeth, too. Thankfully, the crank splines still look fine.