Bugs and non-public API shouldn't be covered. If you were supposed to do X but you did Y, it's OK to fix that and break clients who rely on Y without bumping your major version. It's nice to keep stuff working when you can do so reasonably, but not a requirement.

These questions are tricky when you're building a popular OS with a lot of third-party software whose users will blame you if they break on a new release, even if the third-party software maker is actually to blame. They're a lot less tricky in a case like this, where you're providing a library for other programmers to consume.