You can improve performance without changing the function's interface. Or remove a bug which causes an unrecoverable error. Or fix a memory leak. Or improve the

You can improve performance without changing the function's interface. Or remove a bug which causes an unrecoverable error. Or fix a memory leak. Or improve the error message on an internal assertion. Or add new methods to an object's interface. These would not change the extant observable behavior for most problem domains (although you could probably reasonably argue that any changes in performance characteristics are potentially breaking changes in e.g. game development), and are thus not "breaking", but are nevertheless useful changes.