Demai, Chapter 3, Mishnah 5. Our Mishnah deals with how one conducts himself with the innkeeper - a woman who owned an inn, with whom guests would deposit their belongings and even their food, expecting to receive it back later.
The ruling of the Mishnah:
One who deposits his food with the innkeeper - "me'aser es shehu nosen lah v'es shehu notel mimenah" - he tithes what he gives her and what he takes back from her. In other words, two obligations of separation rest upon him:
At the time of giving: Even though this is not a sale but merely a deposit, and the food remains his, he must separate tithes from what he gives her.
At the time of taking: Even when he receives his food back from her hand, he must tithe again.
And the reason for this - "mipnei shehi chashudah l'chalof" - because she is suspected of switching: the innkeeper is suspected of exchanging his products for her own, and since she is among those suspected regarding tithes, her produce is considered demai. Therefore, when he lets the food out of his hands, the responsibility rests upon him to separate - for a chaver does not let anything untithed out of his possession. And likewise when he receives the food back, lest she exchanged it for her own products.
The reasoning of the first Tanna:
This opinion assumes that the innkeeper's intent in switching the food is for the guest's benefit: she wishes to make sure he receives good food, and since it appears to her that her own food is of better quality, she substitutes it for his. Since she is acting for his benefit, we must be concerned about her, and therefore the one depositing is required to separate tithes in advance - even before he gives her the food.
The view of Rabbi Yosei:
Rabbi Yosei said: "ein anu achra'in l'rama'in" - we are not responsible for cheats, meaning our responsibility does not extend to swindlers and lawbreakers who exchange other people's property. In his opinion, "eino me'aser ela mah shehu notel mimenah" - he tithes only what he takes from her: he is not obligated to separate tithes before giving out of concern that she will take for herself what she is not permitted to take, but only from what he takes back from her, since perhaps it was switched for her products, and in that case the obligation rests upon him.
The basis of his disagreement: Rabbi Yosei holds that the innkeeper does not switch for the guest's benefit, but rather because she is a thief. Therefore we need not be concerned about her, and the one depositing is not required to go so far as to make sure that the thief receives specifically tithed food - that responsibility belongs to the thief, who had no right at all to take what is not his.
In summary: The Tannaim disagree about the reason for the suspicion that the innkeeper switches. According to the first Tanna she switches for the guest's benefit, and therefore we must be concerned about her and tithe both what is handed to her and what is taken from her. And according to Rabbi Yosei she switches out of her thievery, and since "we are not responsible for cheats" - he tithes only what he takes back from her.