I've only heard of lowers breaking (during use) on the old M16 and M16-A1s breaking at the back end when doing bayonet training. I don't know if the military has been able to break the newer reinforced A2 lowers.
I've seen an lower with a broken off ear of the triggerguard (this was on a cast Essential Arms lower). (if you don't support the ear when hammering in the roll pin, you can break it).
I've also seen transferable full-auto lowers that had the hammer pins go out of round. There are a few Class II manufacturers that can repair these.
Chief Armorer for Elite Shooting Sports in Manassas VA
Chief Armorer for Corp Arms (FFL 07-08/SOT 02)
Bookmarks