For the source text click/tap here: Bava Metzia 41
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Steinsaltz "The Mishna (40b) teaches about cases where a shomer (someone paid to watch an object for his friend) accidentally breaks the object.
As explained in the Gemara on our daf, the Mishna opens by teaching that if a havit – a jug or a barrel – falls from the shomer‘s hand and he had picked it up planning to use it, he would be held liable, but if he picked it up to protect it, he would not be responsible. If, however, it broke after he put it down, he would not be held responsible under any circumstances.
According to the second clause of the Mishna, whether it fell from his hand or broke after he replaced it, he will be held liable if he planned to use it but will be free of responsibility if he moved it in order to protect it.
Our Gemara explains the discrepancy in the Mishna by saying that the first half of the Mishna follows the opinion of Rabbi Yishmael, who believes that once someone returns his theft he is no longer responsible for it, even if its owner was unaware that it was returned.
The second half of the Mishna follows the opinion of Rabbi Akiva who believes that a thief remains responsible for the object until he informs the owner that he has returned it.
This forces us to explore authorial structure of the Mishnah and look to the prior texts before the final redaction of Rebbe. We delve once more into the Akiva/Ishmael differences in exegesis.