Bava Kamma 14A

ולא קבל עליו שמירת נזקיו.

but he did not undertake the responsibility of safeguarding the lender’s ox to prevent it from causing damage. Accordingly, the lender remains responsible for his ox, and if it caused damage to the borrower’s ox the lender is liable, as in any other case in which his ox caused damage.

אי הכי, אימא סיפא: נפרצה בלילה או שפרצוה לסטים, ויצתה והזיקה ⁠– פטור. הא ביום ⁠– חייב? הא לא קבל עליו שמירת נזקיו!

The Talmud asks: If that is so, say, and try to explain accordingly, the latter clause of the baraita: If the ox was safely enclosed and a wall broke open at night, or if bandits broke it open and the ox went out and caused damage, the bailee is exempt from liability. The Talmud infers: If it broke open during the day and then caused damage, he is liable. The Talmud asks: According to the interpretation offered in explanation of the previous clause, the baraita concerns a case in which one borrowed an ox but did not undertake the responsibility to prevent it from causing damage. If so, how can the latter clause imply that there are cases in which he is liable? If he did not undertake the responsibility of safeguarding the lender’s ox to prevent it from causing damage, he should never be liable for the damage that it caused.

הכי קאמר: אם קבל עליו שמירת נזקיו ⁠– חייב. נפרצה בלילה או שפרצוה לסטים, ויצתה והזיקה ⁠– פטור.

The Talmud answers that the latter clause of the baraita concerns a different case than that of the previous clause. This is what the latter clause of the baraita is saying: If, by contrast, the borrower did undertake the responsibility of safeguarding the lender’s ox to prevent it from causing damage, if it causes damage, the borrower is liable. But if the ox was safely enclosed and the wall enclosing it broke open at night, or if bandits broke it open and the ox went out and caused damage, he is exempt.