This might seem obviously “yes” at first, but consider a method like foo.debugRepr() which outputs the string FOO and has documentation which says it is meant only to be used for logging / debugging. Then you make a new release of your library and want to update the debug representation to be **FOO**.
Based on the semantics of debugRepr() I would argue that this is NOT a breaking change even though it is returning a different value, because it should only affect logging. However, if someone relies on this and uses it the wrong way, it will break their code.
What do you think? Is this a breaking change or not?
“if someone relies on this and uses it the wrong way”
The wrong way for you might be the perfect solution for someone else. Once things are being used, you have no idea how people will use them, and they will likely use them in ways you didn’t anticipate.
If you go using code in a way contrary to its documentation, you can’t expect semantic versioning to have semantic value to you.
Nothing in there stops you. You are perfectly free to hack anything in your code. But it’s completely outside of your relationship with the author, and frees him from any problem an update may cause you.