הִתְחִיל פָּסוּל לִשְׁחֹט וְגָמַר הַכָּשֵׁר, אוֹ שֶׁהִתְחִיל הַכָּשֵׁר וְגָמַר הַפָּסוּל, פְּסוּלָה. בַּמֶּה דְּבָרִים אֲמוּרִים, שֶׁהִתְחִיל הַפָּסוּל בְּדָבָר שֶׁעוֹשֶׂה אוֹתָהּ נְבֵלָה, כְּגוֹן בַּוֵּשֶט אוֹ בְּרֹב הַקָּנֶה, אֲבָל אִם הִתְחִיל הַפָּסוּל בַּחֲצִי קָנֶה וְגָמַר הַכָּשֵׁר, כְּשֵׁרָה.
If an invalidated person starts to slaughter and when he finishes he is no longer invalidated, or if someone begins to slaughter as a kosher Jew and by the end of the slaughter has become and invalidated person.
What do we tell them? (We say)
From the beginning you were invalidated with respect to an issue, this makes what you slaughtered a nevilah, for example if the gullet and the majority of the trachea (were cut when he found out it is not allowed), however if from the beginning someone is invalidated and half of the Trachea (is already cut) and he finished as a kosher jew, it is permissible.