If a person is fasting and unintentionally eats or drinks, their fast remains valid in most Islamic teachings. This is based on the saying of the Prophet Muhammad (peace be upon him):

“If somebody eats or drinks forgetfully, then he should complete his fast, for what he has eaten or drunk has been given to him by Allah.” (Sahih al-Bukhari and Sahih Muslim)

This ruling applies to acts done out of genuine forgetfulness, meaning the person was not aware they were fasting at the time. Once they realize they are fasting, they should immediately stop eating or drinking and continue their fast as normal.