Bava Kamma, Chapter 8, Mishnah 4. The Mishnah before us discusses another set of interesting and unusual cases, and its wording can be misleading. We will therefore preface with a general explanation, and afterward we will examine the text of the Mishnah itself. We are presented with two categories of people, and in both there is an asymmetry that the Mishnah calls "a bad encounter": an altercation with them is bad, since one who injures them is obligated to pay, whereas they themselves do not pay.
The First Category - Those Lacking Understanding:
A deaf-mute, an intellectually disabled person, and a minor are exempt from the mitzvos, and therefore they are not subject to the obligation of payments at all. One who injures them is obligated in payments, but if they injure others, they are exempt.
The Second Category - A Woman and a Slave:
Regarding them as well it is said that "an encounter with them is bad," and with the same practical result, but the reason is completely different. A woman and a slave are fundamentally obligated in payments, but under normal circumstances, they do not have the money to pay: a woman's assets are subject to her husband's rights, and a slave has no money of his own at all. Since there is no one from whom to collect, one who gets into an altercation with them loses - he will pay them, and they will not pay him.
The difference between the two categories is revealed later on: if the woman is divorced and collects her kesubah, or if money comes into her possession in another way, and similarly if the slave is freed and money comes into his possession, they are obligated to pay the injured party and compensate him in full. This is not the case with those lacking understanding: they were exempt at the time of the action, and therefore even if the intellectually disabled person recovers and regains his understanding, he is not obligated to pay for his damages.
There is a discussion regarding a minor who injured another: once he reaches the age of bar or bas mitzvah, is he required to pay? Although there are opinions that he is obligated, the general consensus is that any payment the minor makes to the injured party after becoming obligated in mitzvos is only to fulfill his heavenly obligation, going beyond the letter of the law, and not as a fundamental legal obligation.
The Text of the Mishnah:
"Hacheresh, hashoteh vehakatan - pegiasan ra'ah" - the three of them lack understanding: the deaf person (a deaf-mute), the intellectually disabled person (one who is mentally impaired), and the minor who has not yet reached the age of mitzvos, and therefore they are exempt from all mitzvah obligations. "Hachovel bahen chayav" - one who injures them is obligated in all payments. (It is worth noting that an intellectually disabled person is not entitled to payments for embarrassment, as he does not truly experience psychological distress, but they are all entitled to the rest of the payments.) "Vehem shechavlu ba'acherim - peturin" - whereas if they injure others they are completely exempt, and even at a later time they will never be obligated to pay, even if their status changes.
"Ha'eved veha'ishah - pegiasan ra'ah" - the slave discussed here is a Canaanite slave, and the woman is any Jewish woman. With them too, an encounter is bad: one who injures them is obligated to pay immediately for any damage caused, "vehem shechavlu ba'acherim - peturin" - whereas they are exempt purely from a practical standpoint, not because they are not obligated, but because they do not have the means to pay: in the case of a woman in most cases, and in the case of a slave in all cases.
"Aval meshalmin le'achar zeman" - their obligation remains intact, and when they have money they will pay: "nisgarshah ha'ishah, nishtachrer ha'eved - chayavin leshalem".
In practice, when a woman or a slave injures a person, the injured party receives a sort of promissory note from the beis din stating that they injured him and became obligated to him for a certain amount, and that he will be able to collect it if and when collection becomes possible. Once the woman collects her kesubah or the slave is freed, the injured party comes to the beis din with his note and claims his debt - and they are indeed obligated to pay.
In summary: In this mishnah we learned about two groups regarding whom it is said that "injuring them is bad": Those lacking understanding - a deaf-mute, a deranged person, and a minor - where one who injures them is liable, but they are exempt forever because they are not subject to liability at all; and a woman and a slave, where one who injures them is liable, but they are exempt only due to a lack of financial means, and therefore they pay at a later time - once the woman is divorced and once the slave is freed.