With respect to the sheared off lug in the bolt's cocking piece cam track - if the cocking piece is placed in half cocked position, and enough force is applied to the bolt handle in an attempt to open it, either that little lug, or the corresponding lug on the cocking piece will fail.
I have seen a number of broken off bolt handles on a variety of rifles - M98 and M96 Mausers, M700 Remingtons, in addition to Lee Enfields. If the bolt cannot rotate (rust on the bolt sleeve threads being the most common reason in the case of the Mausers and Remingtons), and if enough force is applied, the handle will snap. On the Remington, the handle may break mid shank, or the brazed joint peeled. On the Mausers, the bolt handle will be broken out at the root, taking part of the bolt body with it.