Never acknowledge mistakes.
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You never fail, right? That's how you became a manager!

You're human, aren't you? That means that you WILL make mistakes, and that those you manage will see them and recognize them as such.

Refusal to acknowledge and accept responsibility for mistakes indicates an inability to learn from them.

Nothing is more devastating to team morale than a manager who "can do no wrong". This is the path to defensiveness, avoidance of responsibility, and backbiting. If you set the tone by never acknowledging your own failures, you can expect the same behavior from your team.

So go ahead, paint a rosy picture of all things. You're only fooling yourself.