While this is probably as intended and the right way would be to use the class inheriting from QIODevice to check if there is an error, sometimes only QIODevice is accessible. Since the message "Unknown error" (can be translated too) hints that there is an error while there might not actually be, it could lead to some confusion.

Perhaps if this behavior of errorString() cannot be changed, maybe there could be another function like hasErrorString() or something which checks if the errorString is set or not.