A couple of years ago, I had a NPM flat bolt fail on me. Resulting damage included powder burns on my face (thank God for shooting glasses), a ruined receiver (slide snapped in half and cracked/bent the slide rail), along with splintering a nice hi-wood NPM stock when the piece of slide departed.
Was able to attribute the failure to a cracked R/H locking lug, as shown in Brian Q's photo. Since then, I carefully check the bolt on every carbine I own, buy, or shoot with a bright light and magnifying glass. I've found 2 other cracked bolts in the process.
These things are old, and may well have had heavy usage.
I was fortunate that I and my shooting buddy standing to my right, were not seriously injured.
But I do love these little weapons!