The meaning of any word or phrase starts with the "user",the speaker or wriiter, and his intended meaning, and ends with the "receiver" and their perceived understanding. So, in saying that, "an honest mistake" would probably mean to most people that someone made a normal human error that humans are known to make, would be common. However, for someone who uses mistakes to hide behind, and possibly
The meaning of an "honest mistake" is that the speaker truly believed in the honesty/truth of their statement. If it turns out that the statement contained factual errors, then the content was mistaken, but the error was provided honestly. There was no intent to deceive. NO individual can always be fully honest and truthful, without making any errors (unless they never speak).