some of these bugs don't show up when testing with a dummy document, or testing in a developer environment. these bugs are quite hard to reproduce. other bugs might trigger in certain conditions on only certain systems.
bit flags cause quite some headaches specially in languages like c and c++ which don't even complaint about such errors. types are another concern in such languages as you can often use wrong type like equating/assigning to/from different sized integers. these are also a nightmare to debug.