Siman 18
The law of checking the knife and its flaws
א הַשּׁוֹחֵט בְּסַכִּין בְּדוּקָה וְנִמְצֵאת פְּגוּמָה, הֲרֵי זֶהנְבֵלָה; אֲפִלּוּ נָגַע בְּעֶצֶם הַמַּפְרֶקֶת, אֵין תּוֹלִין שֶׁנִּפְגְּמָה בּוֹ אַחַר שְׁחִיטָה, אֶלָּא חַיְישִׁינָן שֶׁמֶּא בָּעוֹר נִפְגְּמָה וְנִמְצָא שָׁחַט בְּסַכִּין פְּגוּמָה, וְאֲפִלּוּ בְּעוֹ
Sief 1
If a slaughterer tested his knife and found it to be defective (what he slaughtered) becomes a nevela; even if the flaw occurred from the neck bone, We don’t attribute the damage to after the slaughter, however we take into consideration that perhaps while it was in the skin the defect we found occurred and the knife was flawed, and this applies also for poultry.