(1) Rashi DH l'Inyan explains that the reason he is liable if the stone inadvertantly fell and caused damage, is based on the Beraisa cited just before by the Gemara, that a human being is always Muad for doing damage, whether he is shogeg or deliberate, coerced or willing. This is based on the concept that a person has a high level of responsibility not to cause damage to others. (Tosfos above 23a DH v'Lichayev states an interesting principle:- a person is required to be more careful not to damage others than he has a responsibility to protect himself from being damaged by others. A person has to think about others more than he thinks about himself!) Therefore when a person has a stone on his lap, he is required to be very careful, because he should realise that a stone is potentionally dangerous if one does not pay attrention. Even if he is not aware that a stone is there it seems that he is still held accountable for not knowing what is on his person.

(2) Yes, you got it right - concerning Shabbat this is considered less than shogeg and in fact is described as "Mitasek" - the person is not intending to perform the action at all.

Shogeg on Shabbat is

(a) where someone knows that it is forbidden to write on Shabbat, for instance, and knows that the action he is doing now is writing but he mistakenly thinks that today is not Shabbat, it is Sunday.

(b) he knows that today is Shabbat and he is aware that he is now writing but he thinks it is permitted to write on Shabbat.

In both the above cases he is deliberately performing the action of writing but he is missing something concerning his knowledge of Shabbat so this is shogeg, not Maizid(intentional).

In contrast, when he does not know he has a stone in his lap and he stands up and the stone travels 4 amos in the public domain as a result of the fall, this is less than shogeg because he is not even aware that he is doing the physical action. Therefore even though a person who did a melacha b'shogeg must bring a sacrifice (in the time when we had a Beit Hamikdash) to atone for his action, nevertheless if he was "Mitasek" he is exempt from a korban because he did not even deliberately perform the action.

The source for the difference between financial laws and shabbat laws is because of the verse (Shemot 35:33) that when they built the mishkan they did this with "melechet machashevet" - "thoughtful work" .Since we learn that any work done to build the mishkan is automatically forbidden on shabbat (this is derived from the fact that at the beginning of Shemot chapter 35 the Torah states the Mitzvah of Shabbat next to the Mitzva of building the mishkan and Rashi 35:2 writes that this teaches that building the mishkan does not override shabbat) it follows that the Torah only forbids melechet machashevet on shabbat. If a person was not intending to do the melacha he is exempt on shabbat, even though concerning the financial aspect he is liable for damage done inadvertantly.

(3) We can see now that there is a significant difference between someone who tripped over the light switch and turned on the light on Shabbat, and between someone who forgot it was shabbat and turned on the light. The person who tripped over was presumably not intending even to the do the action and therefore is mitasek and does not have to bring a sacrifice. In contrast the person who forgot it was shabbat is shogeg and would have to bring a korban and requires forgiveness.

KOL TUV and Chag Sameach

Dovid Bloom