Overconfidence can erode trust.
By default, we assume "if they sound confident then they know what they're talking about."
Most people feel shame if they are talking confidently about something they don't know.
If you are overconfident in one domain, people will update their priors for you.
That leads to people being less willing to trust what you have to say.
Another test: how do you respond when someone points out you are wrong?
Do you graciously acknowledge the error, or do you try to pretend like it didn't happen or wasn't an error in the first place?