By krias HaTorah on Chol HaMoed Pesach after leining the first 3 aliyahs before hagbah on the first Sefer Torah the bal koreh began to say Kaddish by mistake. Should he finish off the kaddish or stop when he realizes the mistake?
He should first finish off Kaddish. After the fourth aliyah, Kaddish should be recited again.
See Iggros Moshe, Orach Chaim 1:101, at length.
The general rule is that once the first amen of Kaddish has been answered, one does not stop in the middle.