Avos chapter 4, mishnah 16 (in some versions, mishnah 13). The author of this teaching is Rabbi Yehudah, one of the five later students of Rabbi Akiva, the five who received semichah from Rabbi Yehudah ben Bava. He lived in the third generation of Tannaim, like the other sages mentioned here - after the Bar Kochba revolt, once things began to settle down. Many Tannaim were in hiding during those years, among them Rabbi Yehudah and also Rabbi Shimon ben Gamliel, the Nasi of that period. As matters quieted, the Sanhedrin was reestablished, not in Yavneh, which had closed, but in Usha - a city about fifteen kilometers (ten mil) southeast of present day Haifa.
Rabbi Yehudah - a portrait:
A native of Usha: Usha was his home city, and he was a prominent figure there and a prominent figure in his generation generally.
'Rosh HaMedabrim' (first among the speakers): This honor was granted to him - the first and leading speaker among the Tannaim of the period, and certainly one of the greatest sages of his time.
The most frequently cited Tanna: Rabbi Yehudah is mentioned in the mishnayos more than any other sage - over six hundred times. Clearly a major Tanna of that era.
The first teacher of Rabbi Yehudah HaNasi: The compiler of the Mishnah first studied under his father, Rabbi Shimon ben Gamliel the Nasi, and afterward went to study under Rabbi Yehudah, and from there went on to study with others as well. Rabbi Yehudah was his first and earliest teacher.
Extreme poverty: The Gemara relates a particularly moving story - his wife made a single cloak of sorts which the two of them shared: when she needed to go out to the market she wore it, and when he went to the beis midrash he wore it. That is how poor they were.
And indeed, those were terrible years for the Jewish people: after the Bar Kochba revolt there was crushing Roman oppression, alongside poverty, famine, drought and locusts - all of these struck that generation in the second half of the second century. These were difficult times.
The wording of the mishnah:
"Hevei zahir b'talmud, sheshigegas talmud olah zadon" - be careful in your learning, for an accidental, unintentional error in the area of talmud is considered as though it were done deliberately and intentionally. To grasp what this means, we first have to understand what learning looked like in that period.
How Torah was learned in those days:
Rabbi Yehudah lived in the generation before the Mishnah, which means that they had nothing in writing. True, individual sages had private notes, but that is not relevant to our discussion - practically speaking, nothing was written down. Learning therefore proceeded in two ways:
Repetition of the traditions: traditions that were transmitted and reviewed from teacher to student. What they reviewed ultimately looked like the mishnayos we have.
Expounding the verses: going verse by verse, and out of this came what we call midrash halachah. Rabbi Yehudah was a very significant figure in this process: the Sifra, also known as Toras Kohanim, the midrash halachah on the book of Vayikra - that is him. Every unattributed, anonymous statement in the Sifra is Rabbi Yehudah.
Everyone, then, reviewed these traditions with nothing written in front of them. To what can this be compared? To songs a person heard on the radio in his childhood: if you ask him to recite them on his own, he will say he does not remember the words; but the moment he hears them playing, within a single second he recalls every last word. And then, as he sings along, he suddenly notices what the words actually say and wonders about them - even though he always knew them, he never gave them any thought and never realized how inappropriate they are, or whatever other story lies behind them.
That is exactly how it was: in their youth the students were a sort of memorization machine, force fed like an ox, in the language of Chazal. Only afterward, once they had absorbed everything, did they open the material up to a degree and examine it. That process - of opening up the tradition, analyzing it, thinking it through, raising difficulties and clarifying the oral tradition - is what is called 'talmud,' and that is what our mishnah is referring to.
This resembles the structure of the Talmud Bavli that we have, which is made up of two parts: the mishnah, and after each mishnah comes the Gemara. The Gemara is what carries out the process of talmud, the process of thinking through that mishnah: what is the source of this mishnah? How do we reconcile it with other traditions and opinions? What is the logic and reasoning behind it? That process is called the process of talmud.
Rabbi Yehudah's warning:
His words are directed at those who teach halachah, at rabbis and dayanim: it is the responsibility of one who issues rulings to make sure that he does not merely memorize the tradition he received and is able to repeat it, but that he understands it - its logic, its basis, how it can be applied and how it ought to be applied. That is the process of Talmud.
Hence the warning: whoever is part of the process of preserving the Oral Tradition - and again, nothing is written down, there is no Mishnah - bears the responsibility, as a talmid chacham, to labor in Talmud and to clarify everything. For if he ultimately errs in his teaching or in his ruling, he cannot excuse himself by saying "oops." There is no such thing in this context: he is regarded as someone who was grossly negligent in his responsibility, and therefore he is a meizid.
Shogeg and meizid:
In matters of responsibility in halachah there are two basic categories:
Shogeg: an unintentional transgression, done with no intent to violate halachah.
Meizid: he knew the thing was wrong, and did it anyway.
Now, someone who fails to engage in Talmud and to clarify the traditions he received orally, and then goes on to teach others, answer questions, rule on matters of halachah and serve as a dayan - if he erred because he did not properly clarify his tradition and did not engage in Talmud as he should have, he has no excuse of "oops, it was an oversight, it was shogeg." The oversight, the accidental and unintended mistake, rises to the level of willfulness: it is considered a deliberate error, because its source is gross negligence.
Since this is his responsibility, one who fails in it bears it in full, for he is in fact creating a distortion or a break in the tradition. A mistaken ruling creates a faulty link in the chain - and even in relation to the streams that disagree and hold a different view, a defective link has now been formed - and this cannot be repaired. In a certain sense it is almost unforgivable, and therefore it is considered willful. Hence the obligation of one who issues rulings to study and clarify everything he has learned, to make sure that he understands it and that he has no remaining questions; and if he does have questions - he must ask them.