Depends on the licence I would say.
If it has a open source licence: Feel free to fork it and fix it. If not you need the permission of the author to post a fix/patch if you use part of his code in it (if the author did not explicitly allow it somewhere).
This is from a legal standpoint (of my understanding, not a lawyer and can differ from country to country), but in reality, I don't think any author has something against a patch. At least I wouldn't
