Sentence:  If we offend against any of the other rules (A's or B's) - that is if we, say, utter the formula incorrectly, or if, say, we are not in a position to do the act because we are, say, married already, or it is the <WSD>purser</WSD> and not the captain who is conducting the ceremony, then the act in question, e.g. marrying, is not successfully performed at all, does not come off, is not achieved. 
Meaning of the word marked with <WSD> tags:  The person responsible for handling the accounts on a ship, or for dealing with the passengers on a ship or aircraft.