It was required that the statements be delivered in set form with verbal accuracy and without correction or stammering. He who failed in a syllable failed in everything, said a legal proverb. The point of this was that the religious-minded Anglo-Saxons put great faith in the supernatural, and they felt that if a man was about to swear falsely, God, by whom he swore, would cause him to falter in his speech.
A web source said that the oath had to be recited without mistakes, "without slip or trip." (That's the keyword to use for a web search on this.)
"Divine irony" is a nice way to summarize the whole event.
I was very interested (even more so because I’m a history buff!) so thanks so much for looking it up for me. And wow, that is really, really something. I’m saving that excerpt.