If you swallow any morsel of food or water deliberately your fast is nullified. If, however, it happened by mistake or accidentally – against your best of intentions – your fast is still considered as valid.
We are accountable for actions within our control; while we are excused for those we have no control. Allah states in the Qur’an: {Allah does not charge a soul except [with that within] its capacity.} (Al-Baqarah 2:286)
The Prophet (peace be upon him) said, “Allah has excused my community of their lapses and slip-ups, their forgetfulness and that which they are coerced into.“