If a person, while being aware that they are fasting, accidentally swallows water during ablution, their fast is broken according to the Ḥanafī school, and they must make up (qaḍāʾ) that day. In fiqh terminology, this is called ḥata’an /accidental eating or drinking meaning that although the person did not intend to swallow, something entered the throat while conscious of being in a state of fasting. Forgetful eating or drinking does not break the fast, but accidental ingestion does, and therefore requires qaḍāʾ.
According to the Shāfiʿī school, if a person, while knowing they are fasting, rinses their mouth normally and water unintentionally reaches the throat, the fast remains valid. However, if the person exaggerates in rinsing and this causes water to reach the throat, the fast is broken