> If you're using a proper type-safe compiled language then most "subtle" breaking changes can't possibly be missed because your code won't compile anymore (assuming you used that API to begin with). You don't need a major version number to call attention to the fact that one parameter of one method changed from a boolean flag to a set of options, anyone who's using that method will find that out pretty quickly.

1. What if it's a dll, .so? You upgrade and find out that your program is broken.

2. Sometimes the API stays the same but the code behind the API changes a result (for example, secure_hash goes from MD5 to bcrypt)?

3. What about non-type safe languages (like HTML or JS, so things like Firefox or Chrome)?

The point is that you should avoid breaking other people's code if you can. What happened if that removal of one function in that one module costs me a full years of work?

Sometimes you can't help yourself. PHP had register_globals. Some people were able to use it safely (initialize all variables before use), but PHP rightfully realized the security implications and disabled it. However, it broke code, and a lot of it.

These are things you should think about and heavily before breaking code. It may be one line for you, but for all the millions of people who use your library it could be thousands of man-years of work.