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Rava stated: Teachers of children, gardeners, butchers, circumcisers, and the town scribe are all regarded as being warned from beforehand (that they will be dismissed if they cause a loss). The general principle is this: If the loss is irrecoverable, the workers are regarded as being warned from beforehand.
Rashi explains that a child, who was taught incorrectly, will always remain with that mistake.
Tosfos asks: Rava himself (in Bava Basra 21a) states that a mistake learned in one’s childhood will most likely be corrected when he becomes an adult! Why then should this be regarded as irreversible?
The Rishonim answer this question by saying that the Gemora in Bava Basra is referring to a teacher who is precise in his studies; however, he is lax and doesn’t see to it that the children properly understand the material. Such mistakes can be corrected at a later date. Here, Rava was referring to a teacher who didn’t understand the material in the first place. Such mistakes will not be easily corrected.
Tosfos explains our Gemora to mean that the child will never be able to recover the time spent learning incorrectly.
The Ran answers that Rava is discussing a teacher who excessively hits his students.
The Chavos Yair does not understand why that would be considered irreversible. [Perhaps the excessive force will make an indelible impression upon the student, and he will have no desire to study when he becomes an adult.]