What's the biggest mistake you've seen someone make after getting promoted?

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I've worked with a lot of leaders stepping into bigger roles, and one pattern keeps coming up. People think they need to prove they're the smartest person in the room, so they talk more, answer everything, and avoid admitting when they don't know something.

Ironically, that often has the opposite effect. The leaders who create the strongest impression tend to speak with more intention, ask better questions and stay composed when they're under pressure.

I'm curious whether others have seen the same thing. What's the biggest mistake you've seen after someone gets promoted?