AAAAARRRRRRRRGGGGGGGGG!!!!!
Okay, here are the three breakage areas:
1. On some early M9s the thin bridge of metal on the alloy frame just above the trigger bar cracked. It didn't impair function, but Beretta made a design change and stress-relieved the area to prevent further trouble.
2. The locking block proved prone to breakage. Usually this happened on guns with silencers attached, as it put weight on the front end of the barrel and stressed the block. I'm not aware of any LB breakages in normal use. Beretta since put stress-relieving corners in the block to reduce the possibility of breakage. LB breakage is NOT what causes the slide to fly off!
3. On some M9s in military use the rear half of the slide broke off right at the locking block cutouts in the slide, at the sharp corners. I'm not aware of any fatalities either, but I did read that the injuries required hospitalization (most likely broken noses or jaws). The official blame was placed on a combination of too much +P+ subgun ammo filtering into the M9 ammo supply, and improper metallurgical controls in some batches of US-made M9s.
Personally, I don't care about any of this because ANY gun will break if abused or stressed. Slides sometimes cracked on military 1911s, as well as the frames. Fortunately, the cracks never resulted in catastrophic failure of those weapons to my knowledge. The 1911 is prone mostly to small parts breakage (barrel link, sear, barrel bushing), which is comparatively minor.