If something that is not normally consumed as a food or medicine is ingested such as paper, fruit seeds etc, then the fast will be deemed invalid and only qaza becomes obligatory, not kaffarah.
However, if a flying insect (such as a housefly), smoke, or dust, enters the throat involuntarily, the fast remains valid, even if it reaches the stomach, as it is difficult to avoid these things. Similarly, when a person crushes a medicine or something else, and its flavour or fumes are felt in the throat, the same ruling applies, and the fast remains valid.