Is one who ate or drank forgetfully while fasting a supererogatory fast, based on an erroneous fatwa, considered to have broken his fast, or should he complete his fast?

The majority opinion is that whoever breaks their fast due to forgetfulness, whether during an obligatory or a supererogatory fast, should complete their fast, and there is no make-up required of them. There is another view which holds that such a person has broken their fast, and their argument is that abstaining from things that break the fast is a pillar of fasting, just like the pillars of prayer. If a fasting person seeks a fatwa from someone whose knowledge they trust and acts upon that fatwa, they are excused. One who intentionally breaks a supererogatory fast does not incur sin, because a person observing a voluntary fast is their own master.