What does Mark Twain's joke "Small mistakes and big mistakes" mean?

The first is to accidentally take someone else's umbrella and forget your own. The second is that I didn't take my own umbrella on purpose, but took someone else's umbrella. From the two answers, I can see what it feels like to make a mistake. Unintentional is a small mistake, intentional is a big mistake.

Small mistakes and big mistakes

Someone asked Mark Twain what is the difference between a small mistake and a big mistake. Mark Twain said, "If you come out of a restaurant, leave your umbrella there and take someone else's umbrella, it's called a small mistake. But if you take someone else's umbrella and leave your umbrella there, it's a big mistake. "