A slaughterer who does not know the halakhot of ritual slaughter is prohibited for consumption, and even if he has slaughtered properly multiple times before observers, one cannot assume his current slaughter is valid since he may unknowingly commit an invalidating error such as interrupting or pressing the knife during the process.
וְאָמַר רַב יְהוּדָה אָמַר שְׁמוּאֵל: כׇּל טַבָּח שֶׁאֵינוֹ יוֹדֵעַ הִלְכוֹת שְׁחִיטָה אָסוּר לֶאֱכוֹל מִשְּׁחִיטָתוֹ, וְאֵלּוּ הֵן הִלְכוֹת שְׁחִיטָה: שְׁהִיָּיה, דְּרָסָה, חֲלָדָה, הַגְרָמָה, וְעִיקּוּר. מַאי קָא מַשְׁמַע לַן? כּוּלְּהוּ תְּנִינְהוּ! לָא צְרִיכָא, שֶׁשָּׁחַט לְפָנֵינוּ שְׁתַּיִם וְשָׁלֹשׁ פְּעָמִים וּשְׁחַט שַׁפִּיר; מַהוּ דְּתֵימָא: מִדְּאִידַּךְ שְׁחַט שַׁפִּיר, הַאי נָמֵי שְׁחַט שַׁפִּיר, קָא מַשְׁמַע לַן: כֵּיוָן דְּלָא גְּמִר, זִימְנִין דְּשָׁהֵי וְדָרֵיס וְלָא יָדַע.
§ And Rav Yehuda says that Shmuel says: With regard to any slaughterer who does not know the halakhot of ritual slaughter, it is prohibited to eat from his slaughter. And these are the halakhot of ritual slaughter: Interrupting the slaughter, pressing the knife, concealing the knife under the windpipe or the gullet in the course of an inverted slaughter, diverting the knife from the place of slaughter, and ripping the simanim from their place before cutting them. The Gemara asks: What is the novelty in what Rav is teaching us? We learned all of them in the mishnayot in the second chapter of this tractate, and therefore it is obvious that a slaughterer who does not know these halakhot is not qualified. The Gemara answers: No, it is necessary in a case where the slaughterer slaughtered before us twice or three times and slaughtered well. Lest you say: From the fact that he slaughtered the other animals well, this animal he also slaughtered well; therefore, Rav teaches us: Since he did not learn the halakhot, sometimes it happens that he interrupts the slaughter or presses the knife, and he does not know that he invalidated the slaughter.