I was trying to go at least one whole day fasting without eating a thing but drinking only water and nothing else, not even juice. But I forgot I was fasting and I ate something so does that make my fasting invalid?
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This is interesting. For me eating breaks your fast. At any time.
This probably shouldn't have been merged with "Is it ok to fast if one had a cup of coffee in the morning?", as eating and drinking are treated separately when fasting. (One can do a fast of no food or drink, a fast of no food and only water, or a fast of no food but beverages allowed).
Also, the whole question really hinges on what you set out to do. If you eat accidentally then it will not count as an all-day fast, but you can still fast the next meal or for a given number of hours.