For balance, I'll take a contrarian view: Some of the more problematic software managers I've worked with or under had been considered excellent developers.

How can being a crack developer hinder you when you're promoted to manager? There are several ways:

Thinking that it's all a technical problem. As individual contributers, we're primarily rewarded for our expertise in solving technical problems. Projects, however, are a mix of technical problems (most of which should be delegated), time and resource management problems, and political problems. If a manager keeps expecting to be rewarded solely for solving the technical problems, they're not going to be effective.

Tunnel Vision. We see things through the filter of our experience. A manager who was successful as a developer using a particular bag of tricks can expect that same bag of tricks to work for a new group under new circumstances, and can block the examination or adoption of new tools or techniques. "Why, back in my day we all made flowcharts and were happy with them. Quit all this talk about UML and get back to work."

Failure to account for individual differences. Developers sometimes have trouble imagining what it's like to live inside someone else's head, and thus have trouble imagining how other people's approaches to problems can be so... wrong different. This can be a hindrance for an individual contributer, but can be a disaster for managers. If you can't understand and work the different viewpoints that members of your team bring to the table, you're liable to commit any one of a number of grevous management sins.

Failure to estimate. A manager who can't get outside of their head, to distance themselves from their past development experiences, risks the sin of estimating on the group's behalf based on how they would do it. This gets compounded when a freshly minted manager is working with a larger team, and doesn't yet fully understand how the added communication overhead is non-linear. "Well, if there were six of me we could do this in six months."