Shabbos, Mishnah 6. This Mishnah teaches us several different cases in which a person who throws an object in a public domain is exempt.
He Remembered After the Object Left His Hand:
The Mishnah deals with someone who threw an object more than four amos in a public domain, and after the object left his hand he remembered that it was Shabbos. The halachah is that a person brings a chatas offering for desecrating Shabbos only when he acted unintentionally, and since in the middle - while the object was still traveling through the air - he remembered that it was Shabbos, he is no longer considered to have acted unintentionally and he cannot bring the offering.
When the Resting Is Not Brought About by His Force:
And what is the law if he did not remember, but rather the object did not come to rest through the force of his throw? The Mishnah lists these cases:
Another person caught the object in midair.
A dog caught it in midair.
It was burned - the fire consumed it before it came to rest naturally.
In all of these he is exempt, and the exemption stems from the resting. In order to be liable, both the lifting and the resting are required together, and the resting - when we are dealing with a throw - must come about through the force and the will of the person who performed the lifting.
The Case of the Dog - a Place of Four by Four:
Regarding the case of the dog, the Gemara explains that even if the dog did not catch it ahead of time, but rather the object fell naturally into its mouth, he is still exempt. For the resting must take place on a spot of four by four - a place measuring four tefachim by four tefachim - and a dog's mouth is not four by four.
Ordinarily this is not a problem when the thrower intends it: had he intended for the object to land in the dog's mouth, his intention would render the dog's mouth as though it were four by four. The rule is that when a person intends for something to come to rest in a particular place, his intention takes the place of the requirement of an area of four tefachim by four tefachim.
Here, however, even though the dog did not catch the object somewhere else but rather in the very spot where it would have landed anyway, since the thrower did not intend for it to come to rest in the dog's mouth, his intention does not help to consider the dog's mouth an area of four by four. This, then, is an additional reason for the exemption, even when the dog does not catch the object ahead of time the way a person does.
One Who Throws in Order to Make a Wound:
The Mishnah continues: one who throws a stone on Shabbos unintentionally in order to make a wound - to injure another in a way that will cause him to bleed - "bein b'adam u'vein b'veheimah" - whether to a person or to an animal, for in both cases making a wound on Shabbos is a melachah. And if "nizkar ad shelo na'aseis chaburah" - he remembered that it was Shabbos before the wound was actually inflicted - he is exempt, since it is required that both the beginning of the act and its end be unintentional.
The Rule Stated in the Mishnah:
From here the Mishnah establishes a rule: "Zeh haklal, kol chayavei chata'os einan chayavin ad shetehei techilasan v'sofan b'shogeg" - anyone who is obligated to bring a chatas is not liable unless both the beginning of the act and its end were done unintentionally. And therefore:
"Techilasan b'shogeg v'sofan b'zadon" - he began unintentionally, and in the end it was done deliberately or he remembered.
"Techilasan b'zadon v'sofan b'shogeg" - he began deliberately, and in the end he forgot that it was Shabbos.
In both of these cases - "peturin", they are exempt, until the beginning and the end are unintentional, until both of them, the beginning and the end together, are unintentional.