Ian McAllister, General Manager at Amazon, answering the question, "What are common mistakes that new or inexperienced managers make?"
Being slow to deal with performance issues - Smoke becomes fire. If you take note of performance issues early you can give gentle corrective feedback. If you're too slow to notice you have to give stronger feedback, and the performance issues may be harder to reverse.
Even seasoned managers make the mistake of being slow to deal with performance issues. It's human nature to want to avoid conflict. A related mistake is not quickly identifying and resolving team issues.
Being slow to resolve team pain points - New managers don't pay attention to or understand their team's pain points. Good managers are always tracking their team's pain points, devising strategies to reduce or resolve them, and then moving on to the next pain point.
Or not quickly identifying and resolving team workload.
Letting the team get swamped - Inexperienced managers keep piling more and more work on the team. Experienced managers either grow the team size to handle the increased load, or deflect the increased work. It takes an experienced manager who's earned the trust of leadership to push back effectively, or to effectively justify why the team needs more headcount.
Check out the thread for many other examples and contributions to the question.