Confucius said: “People define themselves by their faults. By observing their faults, you will understand the true extent of their goodness.”
There are a number of different interpretations of this passage due to the ambiguity of the text. I take it to mean that people only reveal their true nature in unguarded moments.
Most of the time we all put on a pretty good show of how wonderful we are: it’s only when we act unconsciously that the ugly truth that comes out!