Before us is Mishnah 4 in chapter 5 of Maseches Gittin, and it contains three topics: the responsibilities of those who manage the property of orphans, the oath of the guardian at the time he hands over the property, and the law of one who causes damage that is not visible to the eye.
The obligation to tithe the orphans' produce:
The Mishnah opens: "Yesomim shesamchu etzel ba'al habayis, o sheminah lahen avihen apotropos - chayav le'aser peiroseihen" - orphans who rely on a householder, or whose father appointed a guardian for them, that person is obligated to tithe their produce. Two situations are presented here:
"Yesomim shesamchu etzel ba'al habayis" - orphans who depend on a man who does not serve as an official guardian, but who in practice acts on their behalf in every way that a guardian would.
"O sheminah lahen avihen apotropos" - the orphans' father appointed a particular man before his death as a full-fledged guardian.
In both cases the one managing the property bears the obligation to separate maaser and terumas maaser on behalf of the orphans, and this is one of the responsibilities that rest upon him.
The guardian's oath:
From here the Mishnah moves to a different topic: "Apotropos sheminahu avi yesomim - yishava; minahu beis din - lo yishava" - a guardian appointed by the orphans' father must swear; one appointed by beis din need not swear. When the orphans come of age and the guardian transfers the property into their hands, the law depends on who appointed him:
Appointed by the orphans' father: the orphans may demand that he swear that he managed the property properly and took nothing from it for himself.
Appointed by beis din: the orphans cannot compel him to swear.
The reason for the distinction: a guardian appointed by the father presumably would not have accepted the appointment unless he stood to gain something from it, or because of the relationship between himself and the father. We therefore assume that he would not have refused to serve as a trustee merely because of an oath waiting at the end of the road. But when beis din approaches a man, they are simply asking him for a favor, and there is no relationship between them. If we were to obligate him to swear when he hands the property over to the grown orphans, he would say, "Why do I need all this?" and would refuse to serve as a guardian at all.
Abba Shaul disagrees and says "chiluf hadevarim" - the reverse is true. When the appointment comes from the father, the guardian is doing the father a favor, and therefore he will not agree to serve if he will be required to swear in the end. An appointment by beis din, on the other hand, is a prestigious position, and he will not decline it even if he will eventually have to swear. For this reason, in Abba Shaul's view, it is precisely the one appointed by beis din whom we can still make swear.
One who causes damage that is not visible:
In its final section the Mishnah deals with one who damages his fellow's property in a way that is not visible to the eye, that is, a spiritual and halachic damage: "Hametamei, vehamedameia, vehamenasech - beshogeg patur, bemeizid chayav" - one who renders another's food tamei, one who mixes Terumah into it, and one who mixes in wine used for idolatry: if he acted unintentionally he is exempt, and if he acted deliberately he is liable. These are the cases:
"Hametamei" - one who touches his fellow's food while he himself is tamei, thereby transmitting tumah to the food on a spiritual level.
"Vehamedameia" - one who mixes Terumah, which a Yisrael may not eat, into his fellow's food.
"Vehamenasech" - one who mixes yayin nesech, wine that was offered to idolatry, into his fellow's wine, thereby rendering it forbidden.
If he did one of these unintentionally and without intent to harm, he is exempt, because this damage is considered damage that is not visible. Although as a rule one who damages his fellow's property is liable even without intent, here, since the damage is not visible, he is exempt by strict law. But if he did it deliberately, out of a desire to harm his fellow, he is liable. And even though by strict law invisible damage carries no liability, the Sages imposed liability for the sake of tikkun ha'olam: we do not want people doing this to one another, and therefore they ruled that he is liable when he acted intentionally.
Kohanim who rendered an offering pigul in the Mikdash:
The Mishnah continues: "Hakohanim shepiglu bamikdash mezidin - chayavin" - kohanim who deliberately rendered an offering pigul in the Mikdash are liable. This refers to kohanim who are bringing other people's offerings, and in the course of the service they made the offering pigul, for example, by performing one of the services assigned to them, from the slaughtering through the sprinkling of the blood and the like, while intending to perform certain acts with the offering or to eat it after its proper time. Such an intention renders the offering pigul and it is disqualified, and one who eats it is liable to kareis.
If the kohanim did this deliberately and with intent, they are liable. Here too the damage is not visible, but the Sages wished to prevent kohanim from acting this way in order to harm others, and therefore they imposed liability whenever it was done intentionally. And even when the offering is a voluntary one, which a person is not obligated to bring, it causes its owner great distress: he sought to bring an offering to Hashem and the opportunity was taken from him, and therefore we hold the kohen liable.
In summary: in this Mishnah we learned three topics: the obligation to separate maaser from the orphans' produce, both where they rely on a householder and where the father appointed a guardian; the dispute between the Tanna Kamma and Abba Shaul as to which guardian must swear when he hands over the property, the one appointed by the father or the one appointed by beis din; and the law of one who causes damage that is not visible - one who renders food tamei, one who mixes in Terumah, and one who mixes in wine used for idolatry, as well as kohanim who rendered an offering pigul in the Mikdash - that if done unintentionally they are exempt, and if done deliberately they are liable for the sake of tikkun ha'olam.