MB 28: From Kedusha - if he made a mistake in "And the Ofanim..." he must restart "And the Ofanim...", and if it was in the concluding paragraph then he must restart that paragraph. Even though we have established later in Siman 126 that the second one should start from the beginning of the blessing in which the first one made the mistake, here it is different, because having answered "Kadosh, Kadosh [Holy, Holy - the first line of Kedusha]", from there on is considered a new blessing.
MB 29: From the start - because we do not divide a blessing into two. All of this was only at the time when they were accustomed to fulfill the duty of saying the blessings of the Shema for others. Today, where the custom is that everyone says the blessings themselves and nobody fulfills his duty with the blessings of the Chazan, if the Chazan got mixed up in "Who created" or any other blessing, apart from the Amidah, the second Chazan may not go back and repeat that which he has already said himself. [And with printed Siddurim, it is practically impossible that a Chazan will need to step down because of confusion over the words.]
Jonathan Chody jonathan@quantime.co.uk
sha-60.01