According to the Mishna (80a), just because a renter did not follow the instructions of the owner, we do not assume that he is responsible for damage. Thus, if someone rented an ox with the understanding that it would plow a mountainous area, and instead he used it in the valley, he will not be responsible if the plow broke, since he is doing something that is easier than what had been agreed upon.
The Gemara on our daf describes someone who rented a donkey and was warned by the owner that he should avoid the path going through Nehar Pekod, since there is water that way. Instead he told him to take the path leading through Neresh where there is no water. The renter did not follow the instructions and took the donkey through Nehar Pekod where the animal died. In his defense, the renter admitted that he had taken the animal through Nehar Pekod, but he claimed that there was no water there, so the path was no worse than the one recommended by the owner. Rava argues that we should believe the renter’s argument since mah lei le-shaker – what does he gain by lying; if he wanted to lie he could have simply claimed that he had gone through Neresh. Abaye rejects this reasoning since mah li le-shaker be-makom eidim lo amrinan – we do not accept the argument of mah lei le-shaker when there are witnesses who testify to the reality of the situation.
Nehar Pekod is a city near the Tigris River, while Neresh is on the Euphrates, south of Sura. Both of these cities were centers of commerce, and their residents were considered particularly intelligent. There were two separate paths to these cities, one towards the north and the other towards the south.