As is narrated by Abu Hurayra, may Allah be pleased with him, Allah's Messenger, peace and blessings be upon him, said:

If somebody eats something forgetfully while s/he is fasting, and remembers that s/he was fasting, then s/he should complete his/her fast, for Allah has made him/her eat and drink. (Darimi, Sawm: 23; Muslim, Siyam: 171)

Allah does not hold humans responsible for the mistakes that they perpetrate forgetfully/obliviously. For in this case humans have no choice. It is the same for fasting, too. If one eats forgetfully even till one is full, or drinks water till one quenches one's thirst, it does not harm fasting as long as one does not remember that one is fasting. As soon as one remembers, one must immediately clean one's mouth.

This rule is also valid for supererogatory fasting, as well as the obligatory fasting. For the states that invalidate both the obligatory and the supererogatory fasting are the same.

The following hadith (saying of the Prophet), which clarifies the subject about supererogatory fasting is narrated:

Ummu Ishaq, may Allah be pleased with her, was present before Allah's Messenger (PBUH). They brought some sippet to Prophet Muhammad (PBUH) in a pot. Zulyadayn, may Allah be pleased with him, was also there. The Prophet gave a bone with meat to Ummu Ishaq, may Allah be pleased with her.

Zulyadayn said: “ Ummu Ishaq, eat it!”

Ummu Ishaq says:

- Then I remembered that I was fasting. My hand was virtually petrified. I could neither stretch my hand nor pull it. Thereupon, Allah's Messenger (PBUH) asked: “What happened to you?

- I said I was fasting and had forgotten that I was.

Zulyadayn said “Did you remember it only after you were full?”

Then, Prophet Muhammad (PBUH) stated:

Complete your fasting. This is only a bounty that Allah granted you.” (Kutub Sitta, a Condensed Translation and Explanation, 9:457)