As a consultant and executive coach, these are 2 of the most common mistakes managers make:
1. Taking back delegated tasks - if a manager delegates a task to an employee, and the employee comes back to the manager because of an issue, the mistake managers make is to then tell the employee what to do. This sets up a pattern of dependency, and the employee will come back to the manager whenever there is a new issue.
Instead, the manager should tell the employee to first think about how he/she would solve the issue, and then come to the manager with both the problem and proposed solution. Then the manager can comment on the solution. By getting the employee to think through problems first, the manager is reducing the employee's dependence on the manager. In the future, it will be more likely that employees will solve new issues on their own.
2. Making decisions by team consensus. A new trend is for managers to get by-in from subordinates for decisions, and only taking decisions that the majority of the group is in favor of. While is it ok to ask employees for their opinions, the manager needs to make the final call. He is responsible for the results.