I picked up a 70's Schwinn Super Sport for $15 a while back. While doing a simple hub bearing adjustment, the locknut broke-simple as that-snapped clear through.
I was putting a bit of force on it (tightening it against the cone), probably much more than would be experienced during normal road riding, but not more than was needed, and I've never had a nut simply snap like that.
The break is perpendicular to the threads, neither the threads nor the bolt were or are stripped, it's the rear axle, the bike was converted into a fixed gear before I got it, and it is a hollow quick-release axle, if any of those are significant.

As I mentioned, it is as old as 40 yrs, so I suppose it could have become brittle in that time. I wouldn't feel safe riding it if I simply replaced the nut, so I'll probably replace the whole axle, cones, and locknuts, but I'd still like to know if this is a common problem. Come to think of it, I also had an 80's Nishiki road bike which had its front derailleur snap similarly when I shifted. It simply broke on the frame clamp, and I wasn't even putting much force into the shift either.

I kept both the nut and the derailleur and can add pictures of both when time allows.