Was There Reliance on Memory?

While preparing for an upcoming conference, I fired off a response to an email--answering the question the way it "usually" was answered for most conferences. The problem was that this conference was difference and my mistake was in reliance upon my memory.

Is there a piece of information an ancestor provided on a document that might have been solely from his memory, given "off the cuff?" And perhaps his or her error was a very honest one, not given to deceive or confuse, but merely done so quickly "off the top of his head."

And 100 years later, his descendant analyzes it do death when it was originally a very simple error.