Most I have seen were from over-gassed carbine length barrels, after the mid gas barrels came out bolt breakage slowed way down. I broke a Colt and 2 Y/Ms back in the 90s at the cam pin hole. A few 7.62x39 and Grendel bolts around 2005-6 at the lugs each side of the extractor.
In the last few years I have noticed a lot of bolts being Nitride treated, I think over time those lugs will compress and headspace will increase to unusable levels.