Accidentally Drinking Water during Ramadan
Question

What is the ruling on one who eats or drinks during the day in Ramadan by mistake?
Praise be to Allah.What happens if you accidentally drink water during Ramadan?
There is no sin on the one who drinks or eats during the day in Ramadan by mistake and his fast is still valid, because Allah says (interpretation of the meaning): 
“Our Lord! Punish us not if we forget or fall into error.” [Al-Baqarah 2:286]
It was narrated in an authentic Hadith from the Messenger of Allah (peace and blessings of Allah be upon him) that Allah said, “Your prayer is accepted.”
 Abu Hurayrah (may Allah be pleased with him) narrated that the Prophet (peace and blessings of Allah be upon him) said: “Whoever forgets when he is fasting and eats or drinks , let him complete his fast, for it is Allah Who has fed him and given him to drink.”
Does having intercourse in Ramadan out of forgetfulness break the fast?
The same applies if he has intercourse , forgetting that he is fasting. His fast is still valid according to the more sound of the two scholarly opinions, because of the verse quoted above and because of this Hadith, and because the Prophet (peace and blessings of Allah be upon him) said: “Whoever breaks his fast in Ramadan by mistake does not have to make it up or offer any expiation.” (Narrated and classed as authentic by Al-Hakim; classed as sound by Al-Albani in Sahih Al-Jami`, 6070.) This wording includes intercourse and other things that break the fast, if the fasting person does them by mistake out of forgetfulness. This is by the mercy, generosity and kindness of Allah, to Whom be praise and thanks.
For more details, please see the following answer: 38023 .
And Allah knows best.