Zayd is a conscientious Muslim. It is the month of Ramadan. Zayd made intention to fast on the night before. He even makes the effort to follow the sunnah and gets up before fajr in order to have a little something (say some water and dates) to help him last through the day. Unfortunately, he later finds out that his clock has been slow and it turns out that at least on a few occasions he had been eating and drinking after fajr had come in, even though he sincerely believed otherwise.
Now take Jamal. Jamal is not so conscientious. He didn't bother getting up before fajr. In fact, he slept through fajr. When he woke up he sort of realized it was Ramadan and so he knew to skip breakfast but other than that he started his day much as any other day. He was absorbed in various mundane tasks and responsibilities until the middle of the day when he would normally have a lunch break. As was his habit, he goes around the corner to his usual lunch spot to order some food. He noticed he was hungrier than usual so he gets the extra large sandwich, a large pop, a side order and some dessert. After his lunch break he goes back to work and an hour after his meal it occurs to him, with the appropriate amount of shock and surprise, that he just had a full meal in the middle of the day during the month of Ramadan.
Now here's the weird part: It is my understanding (based on the fiqh books I've read) that the slow clock doesn't constitute an excuse so Zayd's fast has been broken and he has to make up the days when he accidentally broke his fast. But Jamal's fast, on the other hand, actually hasn't been broken and that he doesn't have to make up any days.
First, it would probably be good to keep in mind that the question of what breaks your fast or not is different from the question of how actions are judged spiritually or not. For example, lying, gossiping and stealing don't break your fast either but that doesn't mean these are acceptable activities to engage in. Also, I would guess that the above issue is related to how scholars of fiqh understood the relationship between intention, minddfulness and responsibility, but I myself don't have a really good understanding of what the reasoning process is. Anyone out there have a good suggestion?